cpu/stm32/periph_usb: block idle power mode

This USB peripheral seems to not be clocked when entering IDLE, at least
on STM32F3.

Given that an increase of power consumption in the order of 10s of mA
is usually of little concern when powered via USB, this just disables
the idle power state for all STM32 MCUs when USB is in use to rather
err on the side of additional power draw, rather than broken USB.

Author: maribu

cpu/stm32/periph/usbdev_fs.c

@@ -354,6 +354,7 @@ static void _usbdev_init(usbdev_t *dev)
     /* Block STOP/STANDBY */
     pm_block(STM32_PM_STOP);
     pm_block(STM32_PM_STANDBY);
+    pm_block(STM32_PM_IDLE);
 
 #if defined(RCC_CFGR_USBPRE)
     /* If `RCC_CFGR_USBPRE` is defined, the USB device FS clock of 48 MHz is