rt2800usb: use rt2x00usb_register_multiwrite() to set key entries

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: 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>