iwlagn: remove iw_mode check for beacon

Since we have the beacon context now, we no
longer need to rely on iw_mode but can check
the beacon context interface's type. However,
that check must be in the work item instead
due to locking constraints (mutex must be
held when dereferencing beacon_ctx pointer).

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