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: NMauro Carvalho Chehab <mchehab+huawei@kernel.org>
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Acked-by: NFelipe Balbi <balbi@kernel.org>
Link: https://lore.kernel.org/r/e9332e31bec9bcead2c7ced2b25462120488ca85.1610610444.git.mchehab+huawei@kernel.orgSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/usb/dwc3/core.h

@@ -964,6 +964,7 @@ struct dwc3_scratchpad_array {
  * @nr_scratch: number of scratch buffers
  * @u1u2: only used on revisions <1.83a for workaround
  * @maximum_speed: maximum speed requested (mainly for testing purposes)
+ * @gadget_max_speed: maximum gadget speed requested
  * @ip: controller's ID
  * @revision: controller's version of an IP
  * @version_type: VERSIONTYPE register contents, a sub release of a revision