brcmfmac: rework firmware event handling code

Handling of firmware event has been reworked into a seperate
code file. The change is needed as firmware event can be received
in interrupt context. Decoupling of the event handling has been
lowered to allow event processing to sleep.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>