brcmfmac: Always use fifo_credits, also for requested credits.

Currently firmware requested credits do not require fifo credits.
From a buffer management point of view this is incorrect. So
firwmware requested credits require also fifo credits before the
packet can be transferred to the host.

Reviewed-by: Arend Van Spriel <arend@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>