mac80211: tx module cleanup.

This patch removes unnecessary parameter in ieee80211_beacon_add_tim() and
removes unneeded definition and assignment for bdev (instance of net_device) in
ieee80211_beacon_get() and in ieee80211_get_buffered_bc()
(all in tx.c).

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>