rt2x00: fix rt2x00usb_regbusy_read() arguments

Add const to 'field' argument of rt2x00usb_regbusy_read()
(all arguments match rt2x00pci_regbusy_read() ones now).

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