media: rc-sysfs-nodes.rst: better use literals

A literal box provides a better visual when pdf and html output
is generated for things like the output of a sysfs devnode.
It alsod matches other conventions used within the media book.

So, use it.

While here, use literals for protocol names.

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