rt2x00: Fix chipset revision validation

The validation of the chipset revision was broken
since for rt2500usb and rt73usb different registers
should be read. When rt2500usb was loaded for a rt73
device it would false think the chipset was correct
because the wrong register was read and validated.

This has been fixed by expanding the check to also
see if the first 4 bits of the revision is not-0
(When reading the wrong register offset the returned
value is usually 0 which can be interpreted as invalid)

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