modem: hl7800: Fix DNS in sleep mode

When sleep modes are used, sockets are restored from NV after boot.
If the network changes IP addresses, restore sockets maintain old network
info in the HL7800 firmware.
On boot, restored sockets should be closed to ensure new sockets
are created with updated network info.

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