realtek: pcs: select OFF mode with no links on SerDes

realtek: pcs: select OFF mode with no links on SerDes

Because the PCS driver keeps track of the number of registered links for
each SerDes now, we now know when there is no link on a SerDes. In this
case, determine to turn off the SerDes in the mode mapper.

Though the phylink subsystem shouldn't attempt to config something
different when no link/port references a Serdes, be on the safe side.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21365
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>