mac80211: Make use of ieee80211_is_* functions in tx status path

Use ieee80211_is_data, ieee80211_is_mgmt and ieee80211_is_first_frag
in the tx status path. This makes the code easier to read and allows us
to remove two local variables: frag and type.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>