usb: dwc3: allocate gadget structure dynamically

The current code uses commit fac32347

 ("usb: udc: allow adding
and removing the same gadget device") as the workaround to let
the gadget device is re-used, but it is not allowed from driver
core point. In this commit, we allocate gadget structure dynamically,
and free it at its release function. Since the gadget device's
driver_data has already occupied by usb_composite_dev structure, we have
to use gadget device's platform data to store dwc3 structure.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Alan Stern <stern@rowland.harvard.edu>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>