iwlagn: change default beacon interval

When the PAN context is active, but unused, it
may still block scans that take more dwell time
than its beacon interval (which is odd). Work
around this problem by using a default beacon
interval of 200 so scans will fit between.

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