zd1211rw: only update HW beacon if new beacon differs from currect

Update HW beacon only when needed. This appears to make device work in AP-mode
(dtim_period=1) somewhat more stable.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>