rt2x00: Fix compile warning

rt2x00usb_vendor_request_large_buff is write-only, so it is
safe to make the argument a const.

Fixes compile warning:
drivers/net/wireless/rt2x00/rt73usb.c: In function 'rt73usb_load_firmware':
drivers/net/wireless/rt2x00/rt73usb.c:916: warning: passing argument 5 of 'rt2x00usb_vendor_request_large_buff' discards qualifiers from pointer target typ

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/wireless/rt2x00/rt2x00usb.h b/drivers/net/wireless/rt2x00/rt2x00usb.h
index ee3875f..3b4a674 100644
--- a/drivers/net/wireless/rt2x00/rt2x00usb.h
+++ b/drivers/net/wireless/rt2x00/rt2x00usb.h
@@ -185,7 +185,7 @@
  */
 int rt2x00usb_vendor_request_large_buff(struct rt2x00_dev *rt2x00dev,
 					const u8 request, const u8 requesttype,
-					const u16 offset, void *buffer,
+					const u16 offset, const void *buffer,
 					const u16 buffer_length,
 					const int timeout);