net: nuvoton: fix build broken for use phy_get_interface_by_name

The original patch is use phy_get_interface_by_name to set interface.
The new patch is use dev_read_phy_mode to replace it.

Signed-off-by: Jim Liu <JJLIU0@nuvoton.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>