[media] vidioc-enumstd.rst: adjust video standards table

This table is too big for LaTeX output, and lacks columns
specs for LaTeX format.

Also, it has a hidden column, as there are some cell spans
with the wrong values.

Fix it, so it can be displayed properly on LaTeX/PDF.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>