arm: ls102xa: use platform data to check Ethernet interface is not SGMII

arm: ls102xa: use platform data to check Ethernet interface is not SGMII

tsec_private should, as its name suggests, be private. In the next
commit, it'll be moved from a publicly available header file to the C
file that requires it. ls102xa currently does not allow us to do that
because it uses the structure.

The flag is actually set if the Ethernet PHY interface is SGMII in
drivers/net/tsec.c, so simply replace the current check with the same
check made in drivers/net/tsec.c to set the flag.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>