sunxi: musb: Return early on VBUS GPIO error instead of on a positive value

This allows printing the error message when VBUS is detected, as it would with
AXP VBUS detect.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>