mac80211: clean up return value of __ieee80211_parse_tx_radiotap

The return type has more than two values, but it can validly
only ever return TX_DROP and TX_CONTINUE, so use a bool
instead of ieee80211_tx_result.

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