mfd_remove_devices would iterate over all devices sharing a parent with
an mfd device regardless of whether they were allocated by the mfd core
or not. This especially caused problems when the device structure was
not contained within a platform_device, because to_platform_device is
used on each device pointer.

This patch defines a device_type for mfd devices and checks this is
present from mfd_remove_devices_fn before processing the device.

Cc: stable@vger.kernel.org
Signed-off-by: NCharles Keepax <ckeepax@opensource.wolfsonmicro.com>
Tested-by: NPeter Tyser <ptyser@xes-inc.com>
Reviewed-by: NMark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>