drivers: modem: socket: sock_fd isn't an index in modem_socket_put()

modem_socket_put() originally took an index as a parameter and was
later swapped to sock_fd as the reference.

The internal code was never updated to reflect that sock_fd isn't an
index -- it's a separate reference generated via z_reserve_fd().

Let's correct the modem_socket_put() logic.

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/18238



Reported-by: Tobias Svehagen <tobias.svehagen@gmail.com>
Signed-off-by: Michael Scott <mike@foundries.io>