mfd: core: 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 mfd_dev_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-mfd-v1-1-e4eef5ed2da8@marliere.net


Signed-off-by: Lee Jones <lee@kernel.org>