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>
diff --git a/drivers/net/wireless/rt2x00/rt2x00usb.c b/drivers/net/wireless/rt2x00/rt2x00usb.c
index 5015448..b34d4b6 100644
--- a/drivers/net/wireless/rt2x00/rt2x00usb.c
+++ b/drivers/net/wireless/rt2x00/rt2x00usb.c
@@ -156,7 +156,7 @@
 
 int rt2x00usb_regbusy_read(struct rt2x00_dev *rt2x00dev,
 			   const unsigned int offset,
-			   struct rt2x00_field32 field,
+			   const struct rt2x00_field32 field,
 			   u32 *reg)
 {
 	unsigned int i;