modem: hl7800: Fix socket error race condition

Using a global error flag could lead to an error on one
socket appearing as an error with a separate socket instance.
Socket errors moved into the socket context.
Continue to use global error flag for commands not
related to sockets.
For TCP sockets closed by the server, don't try and close
the socket again when offload_put is called.

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