iwlwifi: mvm: fix frame drop from the reordering buffer

An earlier patch made sure that the queues are not lagging
too far behind. This means that iwl_mvm_release_frames
should not be called with a head_sn too far behind NSSN.

Don't take the risk to change completely the entry
condition to iwl_mvm_release_frames, but don't update
the head_sn is the NSSN is more than 2048 packets ahead
of us. Since this just cannot be right. This means that
the scenario described here happened. We are queue 0.

	Q:0				Q:1
	head_sn: 0    -> 2047
					head_sn: 2048

	Lots of packets arrive:
	head_sn: 2047 -> 2150

					send NSSN_SYNC notification

	Handle notification
	from the firmware and
	do NOT move the head_sn
	back to 2048

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>