staging: gdm724x: constantify the struct device_type usage

Since commit aed65af1 ("drivers: make device_type const"), the
driver core can properly handle constant struct device_type. Move the
wwan_type variable to be a constant structure as well, placing it into
read-only memory which can not be modified at runtime.

Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Link: https://lore.kernel.org/r/20240217-device_cleanup-staging-v1-1-286479d9a3f0@marliere.net


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>