rt2x00: Fix chipset detection for rt73usb

The lower 4 bytes of the chipset revision must contain
a non-zero value. This bug was introduced by
"rt2x00: Simplify rt2x00_check_rev".

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