V4L/DVB: v4l2-subdev.h: fix enum_mbus_fmt prototype

enum_mbus_fmt received an index argument that was defined as an int instead
of an unsigned int. This is now fixed. This had the knock-on effect that the
index argument in the callback get_formats in soc_camera.h also had to be
changed to unsigned int.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>