iwlwifi: don't mess up QoS counters with non-QoS frames

In my AMPDU rework, I rely on the sequence numbers of frames. But
I didn't check that the frame has a valid tid before updating the
tracking counters. As a result, the Tx queues were stalled. People
who hit this bug saw that we simply didn't let any data out.

This bug was introduced in 3.3.

This patch fixes that and checks that the frame is a QoS frame before
looking at its tid and changing the counters.

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>