usb: dwc3: gadget: fix gadget suspend/resume

Instead of trying hard to stay connected to the
host, it's best (and far easier) to disconnect from
the host already.

Anything relying on KEEP_CONNECT will just have that
ignored, but we don't have proper hibernation
implementation yet, so there are no regressions.

In any case, hibernation is only useful for runtime
PM, not system sleep.

While at that, also remove dwc3.dcfg which has been
rendered unnecessary.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>