usb: dwc3-generic: fix CONFIG_DM_REGULATOR-off case

When DM_REGULATOR is disabled, all calls will return -ENOSYS. Account
for that so that targets like the IOT2050 will work again.

Fixes: de451d5d5b6f ("usb: dwc3-generic: support external vbus regulator")
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>
1 file changed