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>
a7379234ba