brcmfmac: Fix crash on cleanup.

When driver gets unloaded due to error situation there is a chance
a packet gets received while fws has already been cleaned up. This
will result in kernel crash. This patch adds a check to avoid this
crash.

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