iwlagn: pending frames musn't be incremented if agg is on

During my works on the transport layer I removed code that updated
a local variable (is_agg) that is needed to keep the pending_frames
count up to date. Fix this.

Also, there should be no way to have a packet with TX_CTL_AMPDU set
while the internal aggregation state machine is not in AGG_ON state.
Add a WARN_ON to ensure that.

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>