realtek: pcs: keep track of number of links per SerDes

realtek: pcs: keep track of number of links per SerDes

Add a field to the rtpcs_serdes structure to keep track of how many
links (aka ports) are used on a single SerDes. This is needed to be
known to map kernel interface modes to SerDes hardware modes properly
(e.g. USXGMII --> USXGMII/10G-QXGMII/XSGMII).

While working in rtpcs_create, optimize referencing the SerDes instance
for cleaner code.

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>