realtek: pcs: rtl931x: map interface mode to hardware mode

realtek: pcs: rtl931x: map interface mode to hardware mode

The PCS driver is intended to use a dedicated hardware mode in the
future because the hardware capabilities and modes of the SerDes do not
align with the PHY_INTERFACE_MODE_* modes.

For the beginning, use the previously introduced generic mode mapping
in the SerDes setup and assign the determined hardware mode to the
corresponding field of a SerDes instance. This allows parallel usage of
both mode representations for the time being, until the phy_interface_t
modes can be dropped from internal functions completely.

Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21184
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>