mfd: core: Fix platform-device id generation

Make sure to always honour multi-function devices registered with
PLATFORM_DEVID_NONE (-1) or PLATFORM_DEVID_AUTO (-2) as id base. In this
case it does not make sense to append the cell id to the mfd-id base and
potentially change the requested behaviour.

Specifically this will allow multi-function devices to be registered
with PLATFORM_DEVID_AUTO while still having non-zero cell ids.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>