phy: usbphyc: use regulator_set_enable_if_allowed for disabling vbus supply

Use regulator_set_enable_if_allowed() api instead of regulator_set_enable()
while disabling vbus supply. This way the driver doesn't see an error
when it disable an always-on regulator for VBUS.

This patch is needed for STM32MP157C-DK2 board when the regulator
v3v3: buck4 used as the phy vbus supply in kernel device tree
is always on with the next hack for low power use-case:

&usbphyc_port0 {
        ...
	/*
	 * Hack to keep hub active until all connected devices are suspended
	 * otherwise the hub will be powered off as soon as the v3v3 is disabled
	 * and it can disturb connected devices.
	 */
	connector {
		compatible = "usb-a-connector";
		vbus-supply = <&v3v3>;
	};
};

Without this patch and the previous update in DT the command
"usb stop" failed and the next command "usb start" cause a crash.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Marek Vasut <marex@denx.de>