drivers: gnss: LCX6G use modem chat runtime API

Update the Quectel LCX6G device driver to use the modem chat
runtime API to modify the dynamic pair chat script.

This change makes the driver safer and more readible by
ensuring safe modification of the members of the modem chat
structures.

Signed-off-by: Bjarki Arge Andreasen <bjarki@arge-andreasen.me>