usb: dwc2: Make sure we disconnect the gadget state

I had seen some odd behavior with HiKey's usb-gadget interface
that I finally seemed to have chased down. Basically every other
time I plugged in the OTG port, the gadget interface would
properly initialize. The other times, I'd get a big WARN_ON
in dwc2_hsotg_init_fifo() about the fifo_map not being clear.

Ends up if we don't disconnect the gadget state, the fifo-map
doesn't get cleared properly, which causes WARN_ON messages and
also results in the device not properly being setup as a gadget
every other time the OTG port is connected.

So this patch adds a call to dwc2_hsotg_disconnect() in the
reset path so the state is properly cleared.

With it, the gadget interface initializes properly on every
plug in.

Cc: Wei Xu <xuwei5@hisilicon.com>
Cc: Guodong Xu <guodong.xu@linaro.org>
Cc: Amit Pundir <amit.pundir@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: John Youn <johnyoun@synopsys.com>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Chen Yu <chenyu56@huawei.com>
Cc: Felipe Balbi <felipe.balbi@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>