mac80211: split ieee80211_txrx_result

The _DROP result will need to be split in the RX path but not
in the TX path, so for preparation split up the type into two
types, one for RX and one for TX. Also make sure (via sparse)
that they cannot be confused.

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