Since struct hw_key_entry is 32-bytes large and is smaller than
CSR cache size (which is 64-bytes large) we can use the standard
rt2x00usb_register_multiwrite() helper to set key entries.

This cleanup is a part of preparations for later code unification.
Signed-off-by: NBartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Acked-by: NIvo van Doorn <IvDoorn@gmail.com>
Acked-by: NGertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>