realtek: dsa: enhance pcs_get_state() for RTL93xx

realtek: dsa: enhance pcs_get_state() for RTL93xx

Currently the SerDes driven SFP ports give strange ethtool readings
on RTL93xx devices. Especially duplex and speed are shown even if
no link is up and running. That leads to confusion because the MAC
reports arbitrary values.

Enhance the readout by refactoring the pcs_get_state() function.
Calculate speed/duplex/pause only if link is detected.

Suggested-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Signed-off-by: Harshal Gohel <hg@simonwunderlich.de>
Signed-off-by: Sharadanand Karanjkar <sk@simonwunderlich.de>
Link: https://github.com/openwrt/openwrt/pull/19575
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>