usb: gadget: f_ncm: set SuperSpeed bulk descriptor bMaxBurst to 15

This improves performance on fast connections. When directly
connecting to a Linux laptop running 5.6, single-stream iperf3
goes from ~1.7Gbps to ~2.3Gbps out, and from ~620Mbps to ~720Mbps
in.
Reviewed-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>

drivers/usb/gadget/function/f_ncm.c

@@ -378,7 +378,7 @@ static struct usb_ss_ep_comp_descriptor ss_ncm_bulk_comp_desc = {
 	.bDescriptorType =	USB_DT_SS_ENDPOINT_COMP,
 
 	/* the following 2 values can be tweaked if necessary */
-	/* .bMaxBurst =		0, */
+	.bMaxBurst =		15,
 	/* .bmAttributes =	0, */
 };