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 <[e-mail]>
Link: https://lore.kernel.org/all/[e-mail]/
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/linux/phylink.h

···
  *
  * Determines the negotiation mode to be used by the PCS, and returns
  * one of:
- * %PHYLINK_PCS_NEG_NONE: interface mode does not support inband
- * %PHYLINK_PCS_NEG_OUTBAND: an out of band mode (e.g. reading the PHY)
+ *
+ * - %PHYLINK_PCS_NEG_NONE: interface mode does not support inband
+ * - %PHYLINK_PCS_NEG_OUTBAND: an out of band mode (e.g. reading the PHY)
  *   will be used.
- * %PHYLINK_PCS_NEG_INBAND_DISABLED: inband mode selected but autoneg disabled
- * %PHYLINK_PCS_NEG_INBAND_ENABLED: inband mode selected and autoneg enabled
+ * - %PHYLINK_PCS_NEG_INBAND_DISABLED: inband mode selected but autoneg
+ *   disabled
+ * - %PHYLINK_PCS_NEG_INBAND_ENABLED: inband mode selected and autoneg enabled
  *
  * Note: this is for cases where the PCS itself is involved in negotiation
  * (e.g. Clause 37, SGMII and similar) not Clause 73.