iwlwifi: don't disable AGG queues that are not enabled

If the BA session is torn down before we had a chance to start it
we shouldn't disable the AGG tx queues that weren't enabled.
This can happen in two cases:

1) We get a delBA before we drained our Tx queues in agg start flow
2) We didn't get the (successfull) addBA response on time

Reported-by: Daniel Chyan <dchyan@princeton.edu>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>