ath9k_hw: increase tx status ring buffer size

AR9003 chips read tx status from ring buffer whose max number of
status descriptor is mininal compared to max number of tx buffers.
On a stress condition, it can be easily overflown which might cause
false tx hung detection. Though increasing number of max status
descriptors consumes more memory, it helps to avoid false positive
chip resets.

Cc: Paul Stewart <pstew@google.com>
Signed-off-by: Rajkumar Manoharan <rmanohar@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>