iwlwifi: follow main beacon interval

It is necessary that the PAN context always
use the same beacon interval as the BSS
context unless it is in dual-station mode,
ie. the PAN context is a station as well,
so implement that.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>