Make GBit switch work on RB2011

This change is required to make the GBit switch work on my Mikrotik Routerboard RB2011UiAS-RM, and I assume that the other RB2011 variants are exactly the same in terms of the switch. I have tested the board without and with the patch and confirm that the GBit ports are not supported at all (i.e. no communication works) with the current version in trunk and that everything works with the patch applied. The test box has been running for a few days with the patch applied, and does not show any performance problems in a test setting. I have not used it with LEDE in production so far, but with a previous turnk version of OpenWRT for many years - with the same patch applied. I therefore have good indication that it is stable.

For the record, the switch chip on my test box is identified as
	switch0: Atheros AR8327 rev. 4 switch registered on ag71xx-mdio.0

The value 0x6f000000 has been taken from the table at https://wiki.openwrt.org/toh/mikrotik/rb2011uias with the previous discussion thread still online at https://lists.openwrt.org/pipermail/openwrt-devel/2014-December/029949.html.
One definite improvement from the older OpenWRT trunk version I have been running in production and current LEDE trunk is that the SFP interface can be kept in the default configuration without excessive kernel messages about it constantly going up and down. I have not yet tested an actual SFP module, though.

Performance seems to be reasonable. Routing between two GBit ports on that switch separated by different VLANs with the default firewall ruleset (and one additional rule two allow traffic between the VLANs), but without NAT, iperf3 results are:
	[ ID] Interval           Transfer     Bandwidth       Retr
	[  4]   0.00-10.00  sec   508 MBytes   426 Mbits/sec  102             sender
	[  4]   0.00-10.00  sec   506 MBytes   425 Mbits/sec                  receiver
With a connection going through NAT (also 2 ports on the same GBit switch, same ruleset, but NAT active), routing performance drops to around 250 MBit/s.
(Note that RouterOS achieves beyond 900 MBit/s on the same hardware with the default rule set and the FastTrack rule active even for NAT, see https://wiki.mikrotik.com/index.php?title=Manual:IP/Fasttrack and http://www.mikrotik.com/download/share/FastTrack.pdf).

Summarizing, I strongly recommend to apply this patch in trunk, so that the GBit switch chip rev. 4 can be supported upstream in the next LEDE release (hopefully soon).

Signed-off-by: René Mayrhofer <rene@mayrhofer.eu.org>

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