rt2x00: rt2x00_rev() should return u32

The "rev" field in chipset definition is an u32,
which means that rt2x00_rev() which returns that field
should be of the same type.

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