iwlwifi: mvm: inc pending frames counter also when txing non-sta

Until this patch, when TXing non-sta the pending_frames counter
wasn't increased, but it WAS decreased in
iwl_mvm_rx_tx_cmd_single(), what makes it negative in certain
conditions. This in turn caused much trouble when we need to
remove the station since we won't be waiting forever until
pending_frames gets 0. In certain cases, we were exhausting
the station table even in BSS mode, because we had a lot of
stale stations.

Increase the counter also in iwl_mvm_tx_skb_non_sta() after a
successful TX to avoid this outcome.

CC: <stable@vger.kernel.org> [3.18+]
Signed-off-by: Liad Kaufman <liad.kaufman@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>