carl9170: fix -Wunused-but-set-variable warnings

tx.c: In function ‘carl9170_tx_accounting_free’:
tx.c:159:28: warning: variable ‘txinfo’ set but not used
tx.c: In function ‘carl9170_tx_status_process_ampdu’:
tx.c:383:27: warning: variable ‘ar_info’ set but not used
tx.c: In function ‘__carl9170_tx_process_status’:
tx.c:626:27: warning: variable ‘arinfo’ set but not used
tx.c: In function ‘carl9170_tx_ampdu_queue’:
tx.c:1324:15: warning: variable ‘max’ set but not used

Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>