subdev-formats.rst: don't use adjustbox on a longtable

adjustbox doesn't work on longtables. Also, this
causes an error on LaTeX in interactive mode.

So, use, instead, a tiny font.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>