drivers: usb: device: stm32: do not manually enable the clock on STM32H7

Do not manually enable the USB clock on STM32H7, as it is already done
in usb_dc_stm32_clock_enable() using the values in the device tree.

This partially fixes the build for STM32H7 devices with a single USB
controller.

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