usb: dwc2: get optional vbus-supply regulator once

Move devm_regulator_get_optional() call to probe routine. This avoids
'vbus-supply' regulator to be requested lots of times, upon each call
to dwc2_vbus_supply_init(), e.g. like with runtime pm.

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>