rt2x00: Pad beacon to multiple of 32 bits.

Pad beacon to a multiple of 32 bits in preparation for the change
from memcpy_toio() to __iowrite32_copy() in register_multiwrite().

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