usb/net: fix ECM driver in composite mode

Since commit b48a8c32 ("usb: device: Add composite status callback")
the cb_usb_status callback is not called anymore in composite mode. The
cb_usb_status_composite callback is called instead.

This patch updates the ECM driver to use either one or the other
depending on CONFIG_USB_COMPOSITE_DEVICE.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>