iwlwifi: clean up some beacon handling

There's no need to check for NULL before
calling dev_kfree_skb() since it is valid
to call it on NULL -- it becomes a no-op.

There's also no need to initialise the
beacon_skb variable to NULL just after
the memory it is in has been kzalloc'ed.

Some minor whitespace cleanups, and a
lock assertion in a function that needs
the mutex (to access the beacon_skb var)
complete the patch.

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