usb/dwc3/gadget: fix kernel-doc parameter warning

Fix kernel-doc warning:

../drivers/usb/dwc3/gadget.c:510: warning: Excess function parameter 'dwc' description in 'dwc3_gadget_start_config'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>