iwlwifi: avoid sending too many commands

When the PAN context is unused, there's no
need to continually update it in the device.
So track which contexts are active (with the
special case that the WLAN context is always
active ...) and only send their commands to
the device when needed.

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