rt2x00: Properly identify rt2800usb devices.

Sitecom WLA4000 (USB ID 0x0df6:0x0060) is an RT3072 chipset.
Sitecom WLA5000 (USB ID 0x0df6:0x0062) is an RT3572 chipset.

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