sunxi: musb: Improve output during probing

When we return an error the usb core will print an error-message, so in this
case do not print anything.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>