phylink: ReST-ify the phylink_pcs_neg_mode() kdoc

Stephen reports warnings when rendering phylink kdocs as HTML:

  include/linux/phylink.h:110: ERROR: Unexpected indentation.
  include/linux/phylink.h:111: WARNING: Block quote ends without a blank line; unexpected unindent.
  include/linux/phylink.h:614: WARNING: Inline literal start-string without end-string.
  include/linux/phylink.h:644: WARNING: Inline literal start-string without end-string.

Make phylink_pcs_neg_mode() use a proper list format to fix the first
two warnings.

The last two warnings, AFAICT, come from the use of shorthand like
phylink_mode_*(). Perhaps those should be special-cased at the Sphinx
level.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/all/20230626162908.2f149f98@canb.auug.org.au/
Link: https://lore.kernel.org/r/20230626214640.3142252-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>