brcmfmac: store revinfo retrieval result

When revinfo retrieval fails we can not show the firmware version
in ethtool driver info. Store the result to be used when handling
ethtool driver info callback.

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: Kalle Valo <kvalo@codeaurora.org>