ext: hal: stm32cube: fix stm32l4xx VDDUSB supply control

The STM32L4x2 SoCs need to control the isolation of the USB features
from VDDUSB. This is done through the PWR_CR2 bit USV. The STM32L4 HAL
in stm32l4xx_ll_pwr.h wrongly checks for the PWR_CR2_PVME1 bit, which
is only available on Cat. 3 devices. Replace the check by PWR_CR2_USV
like it is already done in stm32l4xx_hal_pwr_ex.c.

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