modem: hl7800: fix PSM

Fix power-save-mode (PSM) operation.
When in PSM, do not bring the networking interface down
when an out-of-coverage event occurs.
When PSM goes into hibernate, this will cause an
out-of-coverage event to occur, even though the device
still has access to service.
Keeping the networking interface in the up state
allows an app to send data whenever it needs to.

Signed-off-by: Ryan Erickson <ryan.erickson@lairdconnect.com>