mfd: max77693: Store I2C device type as enum and add default unknown

Store the device type (obtained from i2c_device_id) as an enum and add a
default type of unknown to distinguish from case when this is not set
at all.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>