net: phylink: make phylink_parse_mode() support non-DT platform

commit aec48912fd0b06f8631fdd549d4035f39753a267 from
https://source.codeaurora.org/external/autobsps32/linux



Certain platform does not support DT, so we make phylink_parse_mode() to
allow non-DT platform to use it to setup in-band AN advertising.

Issue: ALB-8243
Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit ab393850)
Signed-off-by: Radu Pirea (NXP OSS) <radu-nicolae.pirea@oss.nxp.com>
Signed-off-by: Zhantao Tang <zhantao.tang@windriver.com>