cmd: Enable BIND by default if we have USB_ETHER

The nature of the network stack means that if we are going to use the
gadget mode USB network driver there's no easy path to implicitly
bind/unbind the driver. Enable the "bind" command by default here so
that we can bind/unbind this as needed.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>