zd1211rw: batch beacon config commands together

Beacon config function writes beacon to hw one write per byte. This is very
slow (usually taking more than 100ms to finish) and causes high CPU usage
when in AP-mode (kworker at ~50% on Intel Atom N270). By batching commands
together zd_mac_config_beacon() runtime can be lowered to 1/5th and lower
CPU usage to saner levels (<10% on Atom).

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