modem: hl7800: Fix TCP server closed interrupting TX/RX
modem: hl7800: Fix TCP server closed interrupting TX/RX

If TCP server closed notification is received it
could interrupt the TCP socket RX or TX data sequence.
Remove the SOCK_SERVER_CLOSED state and instead
use a unique error code to know if the socket
has been closed by the server.

Signed-off-by: Ryan Erickson <445db09b136432317bdbb8e4308a30651390c2e2@lairdconnect.com>

finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr