media: mc-device: remove unnecessary __must_check

In the file mc-device.c, the function
media_device_register_entity_notify() does not need to have the
__must_check attribute since it returns only a value of 0.
Therefore, we can remove this attribute and change the function's
return type.

Signed-off-by: Jason Kim <sukbeom.kim@gmail.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>