iwlwifi: update LQ for bcast station on channel change

The rate table in the bcast LQ is computed only when the station is
allocated, and chooses the lowest rate for the band. Because of when this
occurs, this is the 2.4 GHz band and uses the 0x420a (CCK, 1 Mbps) rate. In 5 GHz
beaconing mode, this rate will prevent beacons from being sent and any other
packets from being received.

We can fix this by re-initializing the bcast station's LQ command when the
channel is changed.

Signed-off-by: Daniel Halperin <dhalperi@cs.washington.edu>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>