zd1211rw: add beacon watchdog and setting HW beacon more failsafe

When doing tx/rx at high packet rate (for example simply using ping -f),
device starts to fail to respond to control messages. On non-AP modes
this only causes problems for LED updating code but when we are running
in AP-mode we are writing new beacon to HW usually every 100ms. Now if
control message fails in HW beacon setup, device lock is kept locked
and beacon data partially written. This can and usually does cause:

 1. HW beacon setup fail now on, as driver cannot acquire device lock.
 2. Beacon-done interrupt stop working as device has incomplete beacon.

Therefore make zd_mac_config_beacon() always try to release device lock
and add beacon watchdog to restart beaconing when stall is detected.

Also fix zd_mac_config_beacon() try acquiring device lock for max 500ms,
as what old code appeared to be trying to do using loop and msleep(1).

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