usb: dwc3: ep0: Don't prepare beyond Setup stage

Since we can't guarantee that the host won't send new Setup packet
before going through the device-initiated disconnect, don't prepare
beyond the Setup stage and keep the device in EP0_SETUP_PHASE. This
ensures that the device-initated disconnect sequence can go through
gracefully. Note that the controller won't service the End Transfer
command if it can't DMA out the Setup packet.

Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/6bacec56ecabb2c6e49a09cedfcac281fdc97de0.1650593829.git.Thinh.Nguyen@synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>