drivers: modem: gsm: Fix set gsm->state in gsm_ppp_stop

The driver should set the `gsm->state` to GSM_PPP_STOP when
`gsm_ppp_stop()` is run. This commit fixes that.

Fixes https://github.com/zephyrproject-rtos/zephyr/issues/51505



Signed-off-by: Jeppe Odgaard <jeppe.odgaard@prevas.dk>