brcmfmac: add handler for credit map firmware events

brcmfmac: add handler for credit map firmware events

The firmware signalling functionality needs the credit map firmware
events. This patch adds registration of a handler for this event.

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