brcmfmac: Accept only first creditmap event.

During P2P testing it turned out that the firmware sents multiple
multiple creditmap event messages. Only the first message from the
firmware should be processed. Otherwise the firmware-signalled flow
control can run haywire when it has packets outstanding in firmware.

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>