zd1211rw: fix beacon interval setup

Vendor driver uses CR_BNC_INTERVAL at various places, one is HW_EnableBeacon()
that combinies beacon interval with BSS-type flag and DTIM value in upper 16bit
of u32. The other one is HW_UpdateBcnInterval() that set_aw_pt_bi()
appears to be based on. HW_UpdateBcnInterval() takes interval argument as u16
and uses that for calculations, set_aw_pt_bi() uses u32 value that has flags
and dtim in upper part. This clearly seems wrong. Also HW_UpdateBcnInterval()
updates only lower 16bit part of CR_BNC_INTERVAL. So make set_aw_pt_bi() do
calculations on only lower u16 part of s->beacon_interval.

Also set 32bit beacon interval register before reading values from device,
as HW_EnableBeacon() on vendor driver does. This is required to make beacon
work on AP-mode, simply reading and then writing updated values is not enough
at least with zd1211b.

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