net: phylink: fix sphinx complaint about invalid literal

sphinx complains about the use of "%PHYLINK_PCS_NEG_*":

Documentation/networking/kapi:144: ./include/linux/phylink.h:601: WARNING: Inline literal start-string without end-string.
Documentation/networking/kapi:144: ./include/linux/phylink.h:633: WARNING: Inline literal start-string without end-string.

These are not valid symbols so drop the '%' prefix.

Alternatively we could use %PHYLINK_PCS_NEG_\* (escape the *)
or use normal literal ``PHYLINK_PCS_NEG_*`` but there is already
a handful of un-adorned DEFINE_* in this file.

Fixes: f99d471a

 ("net: phylink: add PCS negotiation mode")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Link: https://lore.kernel.org/all/20230626162908.2f149f98@canb.auug.org.au/
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>