net: wwan: iosm: release data channel in case no active IP session

If there is no active IP session (interface up & running) then
release the data channel.

Use nr_sessions variable to track current active IP sessions.
If the count drops to 0, then send pipe close ctrl message to
release the data channel.

Signed-off-by: M Chetan Kumar <m.chetan.kumar@linux.intel.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>