ath79: mikrotik: fix beeper phantom noise on RB912

Analysis done by Denis Kalashnikov:

It seems that some ROS versions on some routerboard models have this bug:
after silence boot (no output to uart, no beeps) beeper clicks when wireless traffic is.

https://forum.mikrotik.com/viewtopic.php?f=3&t=92269
https://forum.mikrotik.com/viewtopic.php?t=63399

From these links:

1)
Hello, I have RB951G-2HnD and I noticed strange thing
when I loaded the device with some wireless traffic it
produced strange sound - like hissing, fizzing etc.

2)
Same problem still on 6.33, with silent boot enabled
I hear buzzing noise on wireless load.

3)
The sound is fixed in v5.19, it was a bug that caused beeper to make clicks.

It also got fixed in RouterOS:

* What's new in 5.19 (2012-Jul-16 10:51):
fix ticking sound on RB411UAHL;

* What's new in 6.38.3 (2017-Feb-07 09:52):
rb3011 - fixed noise from buzzer after silent boot;

I've checked with an oscilloscope that:
* When on the ssr beeper pin is 0,
  on the beeper itself is 1 (~5V),
  and when on the ssr beeper pin is 1,
  on the beeper is 0
  The beeper doesn't consume power,
  so 1 should be a default/idle value for the ssr beeper pin).
* When there is wireless traffic (ping packets)
  in the background and the beeper clicks, I see
  pulses on the beeper itself,
  but no pulses on the ssr beeper pin (Q5 pin of 74hc595).
  When I manually toggle the ssr beeper pin I see pulses on both.
  So, it is likely that the phantom beeper clicks are caused by the EMI.

Suggested-by: Denis Kalashnikov <denis281089@gmail.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>
(cherry picked from commit a58bcc9e673db3c6aa39f2089d216d51c8356418)

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