drivers/modem: add user_data to modem_cmd_handler_data

added user_data field to modem_cmd_handler_data

this allows for multiple instances of modem drivers which use
the modem cmd handler. currently, the only identifiable
parameter passed to the modem command handlers is the
modem_cmd_handler_data struct.

The added user_data variable allows for the a modem driver to
pass its dev or data pointer to the modem_cmd_handler_data
struct to be retrieved from within the modem command callbacks.

Signed-off-by: Bjarki AA <[email protected]>

Author: Bjarki AA

drivers/modem/modem_cmd_handler.h

@@ -118,6 +118,9 @@ struct modem_cmd_handler_data {
 	/* locks */
 	struct k_sem sem_tx_lock;
 	struct k_sem sem_parse_lock;
+
+	/* user data */
+	void *user_data;
 };
 
 /**