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>