brcmfmac: add credit borrowing mechanism

The firmware provides credits to the driver per WMM-AC. When
only AC_BE are to be transmitted to the firmware the driver
may use credits from other priorities to send AC_BE packets
towards the firmware.

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>