ath5k: Fix IMR init

*On a previous patch i splitted AR5K_INT_TX to multiple different TX
interrupt flags for better handling but i forgot to unmask the new
TXDESC and TXEOL interrupts on ath5k_init and only left TXOK. However
for each queue we enable TXDESC and TXEOL interrupts, not TXOK so we don't
handle TX interrupts at all (because these interrupts remain masked on
PISR) and under load it results packet loss. Fix the problem by
unmasking TXDESC and TXEOL on ath5k_init.

Signed-Off-by: Nick Kossifidis <mickflemm@gmail.com>

Signed-off-by: John W. Linville <linville@tuxdriver.com>