ath9k_hw: Fix TX interrupt mitigation settings

TX interrupt mitigation reduces the number of interrupts
by addressing several interrupt actions (AR_IMR_TXOK,
AR_IMR_TXDESC) all in one interrupt so when enabling
it discard setting the other interrupts.

Without this TX interrupt mitigation would actually
increase the number of interrupts two-fold. We still
leave TX interrupt mitigation disabled as it is still
being tested.

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