rt2x00: Fix optimize register access for rt2800pci

The patch
	rt2x00: Optimize register access in rt2800pci
from Helmut Schaa missed one register call, namely
the rt2800_register_multiwrite which should be changed
to rt2x00pci_register_multiwrite.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>