brcmfmac: do not use internal roaming engine by default

Some evidence of curing disconnects with this disabled, so make it a default.
Can be overridden with module parameter roamoff=0
See: http://projectable.me/optimize-my-pi-wi-fi/

Signed-off-by: Phil Elwell <phil@raspberrypi.org>
---

Index: backports-v4.18-rc7/drivers/net/wireless/broadcom/brcm80211/brcmfmac/common.c
===================================================================
--- backports-v4.18-rc7.orig/drivers/net/wireless/broadcom/brcm80211/brcmfmac/common.c
+++ backports-v4.18-rc7/drivers/net/wireless/broadcom/brcm80211/brcmfmac/common.c
@@ -70,7 +70,11 @@ static int brcmf_fcmode;
 module_param_named(fcmode, brcmf_fcmode, int, 0);
 MODULE_PARM_DESC(fcmode, "Mode of firmware signalled flow control");
 
+#if defined(CONFIG_ARCH_BCM2835)
+static int brcmf_roamoff = 1;
+#else
 static int brcmf_roamoff;
+#endif
 module_param_named(roamoff, brcmf_roamoff, int, 0400);
 MODULE_PARM_DESC(roamoff, "Do not use internal roaming engine");