usb: gadget: ether: populate _reset_ callback

populate _reset_ callback to the USB ethernet gadget since UDC core
expects every gadget driver to have the reset callback. This shouldn't
be needed once the ethernet gadget driver is adapted to use the
composite driver.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>