net: dm: fec: Obtain the transceiver type from the DT

The DT property "phy-mode" already provides the transceiver type.
Use it so that we do not have to also set CONFIG_FEC_XCV_TYPE

Signed-off-by: Martin Fuzzey <martin.fuzzey@flowbird.group>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>