iwlagn: track beacon interval sent to device

Sometimes, when mac80211 changes the beacon
interval or when it isn't yet set in mac80211
before association, the uCode will sysassert
because we send it confusing RXON timing vs.
PAN parameters. To fix this, track the last
beacon interval sent to the device and use
that in PAN parameter calculations.

This fixes a bug during P2P group formation
as a client (and possibly association to a
regular AP) while connected to another AP.

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