USB: dwc3: document gadget_max_speed

This new field was added to struct dwc3_scratchpad_array, but
a documentation for it was missed:

	../drivers/usb/dwc3/core.h:1259: warning: Function parameter or member 'gadget_max_speed' not described in 'dwc3'

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Felipe Balbi <balbi@kernel.org>
Link: https://lore.kernel.org/r/e9332e31bec9bcead2c7ced2b25462120488ca85.1610610444.git.mchehab+huawei@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>