brcm80211: fmac: use correct firmware filename

Also don't use so generic BRCMF_USB_FW_NAME as we may need different
firmwares in the future.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>