usb: gadget: udc: renesas_usb3: Fix b-device mode for "workaround"

If the "workaround_for_vbus" is true, the driver will not call
usb_disconnect(). So, since the controller keeps some registers'
value, the driver doesn't re-enumarate suitable speed after
the b-device mode is disabled. To fix the issue, this patch
adds usb_disconnect() calling in renesas_usb3_b_device_write()
if workaround_for_vbus is true.

Fixes: 43ba968b

 ("usb: gadget: udc: renesas_usb3: add debugfs to set the b-device mode")
Cc: <stable@vger.kernel.org> # v4.14+
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>