iwlwifi: use maximum aggregation size

Use the values from the peer to set up the ucode
for the right maximum number of subframes in an
aggregate. Since the ucode only tracks this per
station, use the minimum across all aggregation
sessions with this peer.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>