usb: dwc2: fix unbalanced use of external vbus-supply

When using external vbus supply regulator, it should be enabled
synchronously with PWR bit in HPRT register. This also fixes
unbalanced use of this optional regulator (This can be reproduced
easily when unbinding the driver).

Fixes: 531ef5eb

 ("usb: dwc2: add support for host mode external
vbus supply")

Tested-by: Artur Petrosyan <arturp@synopsys.com>
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>