drivers: usb: device: stm32: only disable the ULPI clock for USB FS

On SoCs with two USB controller, Zephyr selects the FS one, and in that
case the ULPI clock needs to be disabled in sleep mode for the
controller to work.

On SoCs with a single USB HS controller, this operation is not needed.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>