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

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.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>