media: v4l2-subdev.h: fix a kernel-doc markup

As reported by Sphinx:

	./Documentation/driver-api/media/v4l2-subdev:490: ./include/media/v4l2-subdev.h:384: WARNING: Unparseable C cross-reference: 'struct'
	Invalid C declaration: Expected identifier in nested name, got keyword: struct [error at 6]
	  struct
	  ------^

The markup there is wrong:
	&struct &v4l2_input -> &struct v4l2_input

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>