ath9k_hw: avoid setting cwmin/cwmax to 0 for IBSS for AR9003

IBSS requires the cwmin and cwmax to be respected when
we reset the txqueues on AR9003 otherwise the distribution
of beacons will be balanced towards the AR9003 card first
preventing equal contention for air time for other peers
on the IBSS.

Without this IBSS will work but only the AR9003 card will be
be issuing beacons on the IBSS.

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