usb: dwc3: st: Fix USB_DR_MODE_PERIPHERAL configuration.

Set USB3_FORCE_VBUSVALID when configured for USB_DR_MODE_PERIPHERAL
mode, as it is required to have a working setup.

This worked on the internal driver by relying on the reset
value of the syscfg register as the bits aren't explicity cleared
and set like the upstream driver.

Also add a comment about what setting this bit means.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>