wireless: rt2x00: rt2800usb: replace X by x

s/X/x

Signed-off-by: Xose Vazquez Perez <xose.vazquez@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>