rt2x00: Fix chipset detection for rt2500usb

The commit below changed the semantics of rt2x00_check_rev so that it no
longer checked the bottom 4 bits of the rev were non-zero.  During that
conversion this part of the check was not propogated to the rt2500usb
initialisation.

    commit 358623c2


    Author: Ivo van Doorn <ivdoorn@gmail.com>
    Date:   Tue May 5 19:46:08 2009 +0200

        rt2x00: Simplify rt2x00_check_rev

Without this check rt73 devices are miss recognised as rt2500 devices and
two drivers are loaded.  Preventing the device being used.  Reinstate this
check.

Signed-off-by: Andy Whitcroft <apw@canonical.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>