brcmfmac: handle firmware signals requesting for packets

The firmware can request the host driver for packets, by sending
either the MAC_REQUEST_CREDIT or the MAC_REQUEST_PACKET primitive.
This patch adds handling of these primitives.

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