drivers: gsm_ppp: replace DT_INST with DT_DRV_INST

Replace error prone DT_INST with proper DT_DRV_INST.

Signed-off-by: Maciej Zagrabski <mzi@trackunit.com>
diff --git a/drivers/modem/gsm_ppp.c b/drivers/modem/gsm_ppp.c
index be4067c..fa91e0f 100644
--- a/drivers/modem/gsm_ppp.c
+++ b/drivers/modem/gsm_ppp.c
@@ -1364,5 +1364,5 @@
 	return 0;
 }
 
-DEVICE_DT_DEFINE(DT_INST(0, zephyr_gsm_ppp), gsm_init, NULL, &gsm, NULL,
+DEVICE_DT_DEFINE(DT_DRV_INST(0), gsm_init, NULL, &gsm, NULL,
 		 POST_KERNEL, CONFIG_MODEM_GSM_INIT_PRIORITY, NULL);