iwlwifi: mvm: clean up broadcast station handling

Unify all the functions that handle the per-interface broadcast
station and make them have mvm and vif parameters. While at it,
add a new function to allocate the broadcast station instead of
open-coding it, and make the combined alloc+send and free+send
functions use the alloc/free & send functions.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>