zd1211rw: let zd_set_beacon_interval() set dtim_period and add AP-beacon flag

Add support for AP-mode beacon. Also disable beacon when interface is set
down as otherwise hw will keep flooding NEXT_BCN interrupts.

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