brcmfmac: add subtype check for event handling in data path

For USB there is no separate channel being used to pass events
from firmware to the host driver and as such are passed over the
data path. In order to detect mock event messages an additional
check is needed on event subtype. This check is added conditionally
using unlikely() keyword.

Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>