drivers: usb: device: stm32: fix a wrong comment STM32H7X and ULPI clock

On STM32H7X SoCs, both USB controllers are HS capable, however it is not
possible to connect an ULPI PHY to the USB2 one, which limit it to FS.
Therefore the ULPI clock in sleep mode has to be disabled for USB2 in
all cases. The ULPI clock for USB1 is already disabled when needed in
the usb_dc_stm32_clock_enable() function like for other SoCs.

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