usb: dwc2: dwc2_vbus_supply_init: fix error check

devm_regulator_get_optional returns -ENODEV if the regulator isn't
there, so if that's the case we have to make sure not to leave -ENODEV
in the regulator pointer.

Also, make sure we return 0 in that case, but correctly propagate any
other errors. Also propagate the error from _dwc2_hcd_start.

Fixes: 531ef5eb ("usb: dwc2: add support for host mode external vbus supply")
Cc: Amelie Delaunay <amelie.delaunay@st.com>
Reviewed-by: Amelie Delaunay <amelie.delaunay@st.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>