iwlagn: use huge command for beacon

When testing some new P2P code, Angie found that the
driver might crash because the beacon command ended
up being bigger than a regular command. This is quite
obvious -- a normal command is limited to roughly 360
bytes but a beacon may be much larger of course.

To fix this, use the huge command buffer.

Reported-by: Angie Chinchilla <angie.v.chinchilla@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>