ath9k: remove pointless sc_txintrperiod and spin_lock_bh on tx prepare

sc_txintrperiod is currently set to 0 and never updated. We won't
be using this, if anything we will consider using TX interrupt
mitigation but that is different and not yet tested. So remove
sc_txintrperiod and the pointless spin_lock_bh() on tx prepare.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>