mac80211: make tx() operation return void

The return value of the tx operation is commonly
misused by drivers, leading to errors. All drivers
will drop frames if they fail to TX the frame, and
they must also properly manage the queues (if they
didn't, mac80211 would already warn).

Removing the ability for drivers to return a BUSY
value also allows significant cleanups of the TX
TX handling code in mac80211.

Note that this also fixes a bug in ath9k_htc, the
old "return -1" there was wrong.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Tested-by: Sedat Dilek <sedat.dilek@googlemail.com> [ath5k]
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com> [rt2x00]
Acked-by: Larry Finger <Larry.Finger@lwfinger.net> [b43, rtl8187, rtlwifi]
Acked-by: Luciano Coelho <coelho@ti.com> [wl12xx]
Signed-off-by: John W. Linville <linville@tuxdriver.com>