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>

drivers/net/wireless/rt2x00/rt2x00usb.c

@@ -124,7 +124,7 @@ EXPORT_SYMBOL_GPL(rt2x00usb_vendor_request_buff);
 
 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)
 {
@@ -134,7 +134,7 @@ int rt2x00usb_vendor_request_large_buff(struct rt2x00_dev *rt2x00dev,
 
 	mutex_lock(&rt2x00dev->usb_cache_mutex);
 
-	tb  = buffer;
+	tb  = (char *)buffer;
 	off = offset;
 	len = buffer_length;
 	while (len && !status) {

drivers/net/wireless/rt2x00/rt2x00usb.h

@@ -185,7 +185,7 @@ int rt2x00usb_vendor_req_buff_lock(struct rt2x00_dev *rt2x00dev,
  */
 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);