rt2800usb:fix efuse detection

The device 057c:8501 (AVM Fritz! WLAN v2 rev. B) currently does not
load. One thing observed is that the vendors driver detects EFUSE mode
for this device, but rt2800usb does not. This is due to rt2800usb
lacking a check for the firmware mode present in the vendors driver,
that this patch adopts for rt2800usb.

With this patch applied, the 'RF chipset' detection does no longer fail.

Signed-off-by: Michael Braun <michael-dev@fami-braun.de>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>