pico-imx7d: Disable USB_ETHER support for bl33 defconfig

For DM_ETH support , it's require to disable this config.
When this config is enable, This generate a error with spl in linker script

Signed-off-by: Joris Offouga <offougajoris@gmail.com>
Reviewed-by: Stefano Babic <sbabic@denx.de>