UPSTREAM: usb: dwc3: st: Fix USB_DR_MODE_PERIPHERAL configuration.

UPSTREAM: 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.

Change-Id: Ibd3a18b3a85b53dcf8471a1231b502185641c010
Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Wu Liang feng <wulf@rock-chips.com>
(cherry picked from commit 27a0faafdca53bda21ed340ca8f8960696dda049)