brcmfmac: assure USB dongle firmware is reset upon module unload

Upon unloading the brcmfmac module the USB firmware should be reset
as the device remains powered. The reset assures a known device
state when a new brcmfmac driver load is being done.

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>