brcmfmac: schedule dequeue upon firmware-signal reception

Several firmware signals should be considered as opportunity to
send packets to the firmware. This patch adds conditional scheduling
of the dequeue worker thread while handling those signals.

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: John W. Linville <linville@tuxdriver.com>