i2c: constify 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
i2c_adapter_type and i2c_client_type variables to be constant structures as
well, placing it into read-only memory which can not be modified at
runtime.

Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>