usb: dwc3: core: Add missing GHWPARAMS9 doc

Add missing documentation for struct dwc3_hwparams new field hwparams9
to avoid kernel doc build warning.

Fixes: 16710380

 ("usb: dwc3: Capture new capability register GHWPARAMS9")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/f4c491f7614e623755fafe640b7e690e7c5634e2.1619471127.git.Thinh.Nguyen@synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>