mac80211: get rid of function pointers in TX path

This changes the TX path to no longer use function pointers for
TX handlers but rather invoke them directly. If debugging is
enabled, mark the TX handlers noinline because otherwise they
all get inlined into invoke_tx_handlers() which makes it harder
to see where a bug is.

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