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>

drivers/net/wireless/rt2x00/rt2500usb.c

@@ -1550,7 +1550,9 @@ static int rt2500usb_init_eeprom(struct rt2x00_dev *rt2x00dev)
 	rt2500usb_register_read(rt2x00dev, MAC_CSR0, &reg);
 	rt2x00_set_chip(rt2x00dev, RT2570, value, reg);
 
-	if (!rt2x00_check_rev(&rt2x00dev->chip, 0x000ffff0, 0)) {
+	if (!rt2x00_check_rev(&rt2x00dev->chip, 0x000ffff0, 0) ||
+	    rt2x00_check_rev(&rt2x00dev->chip, 0x0000000f, 0)) {
+
 		ERROR(rt2x00dev, "Invalid RT chipset detected.\n");
 		return -ENODEV;
 	}