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: NArtur Petrosyan <arturp@synopsys.com>
Acked-by: NMinas Harutyunyan <hminas@synopsys.com>
Signed-off-by: NFabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: NAmelie Delaunay <amelie.delaunay@st.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>