docs: net: sja1105.rst: fix table format

docs: net: sja1105.rst: fix table format

There's a table there with produces two warnings when built
with Sphinx:

    Documentation/networking/dsa/sja1105.rst:91: WARNING: Block quote ends without a blank line; unexpected unindent.
    Documentation/networking/dsa/sja1105.rst:91: WARNING: Block quote ends without a blank line; unexpected unindent.

It will still produce a table, but the html output is wrong, as
it won't interpret the second line as the continuation for the
first ones, because identation doesn't match.

After the change, the output looks a way better and we got rid
of two warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>