ath9k_hw: Fix memory leak on ath9k_hw_rf_alloc_ext_banks failure

The allocated externel radio banks have to be freed in
case of ath9k_hw_rf_alloc_ext_banks failure.

Cc: stable@kernel.org
Signed-off-by: Rajkumar Manoharan <rmanoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>