counter: constify the struct device_type usage

counter: constify the struct device_type usage

Since commit aed65af1cc2f ("drivers: make device_type const"), the driver
core can properly handle constant struct device_type. Move the
counter_device_type variable to be a constant structure as well, placing it
into read-only memory which can not be modified at runtime.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: "Ricardo B. Marliere" <ricardo@marliere.net>
Link: https://lore.kernel.org/r/20240219-device_cleanup-counter-v1-1-24d0316ae815@marliere.net
Signed-off-by: William Breathitt Gray <william.gray@linaro.org>