ath9k: remove useless NULL initialisers

When I added .set_frag_threshold I didn't realise it was already
there which now generated a sparse warning. Therefore, remove
the .set_frag_threshold NULL initialiser, and while at it all the
other useless ones.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>