rt2x00: Restrict firmware file lengths

Add extra security to the drivers for firmware loading,
check the firmware file length before uploading it to
the hardware. Incorrect lengths might indicate a firmware
upgrade (which is not yet supported by the driver) or
otherwise incorrect firmware.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>