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 <baa@trackunit.com>