mac80211: simplify ieee80211_work_work

Since local->tmp_channel is always NULL in one branch, some code paths
will newer be taken in that branch, so remove them.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Reviewed-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>