usb: gadget: dummy_hcd: set no_sg_constraint only on the SuperSpeed bus

dummy_hcd registers two separate root hubs/buses: a HighSpeed one
emulating an EHCI-class controller, and a SuperSpeed one emulating an
xHCI-class controller.  usb_submit_urb() rejects SG URBs whose
non-final segments are not a multiple of the endpoint maxpacket size,
unless the bus has no_sg_constraint set.

For SuperSpeed bulk endpoints maxpacket is 1024 bytes; usbtest
generates SG lists with varying segment sizes (e.g. 512 bytes) that
are valid transfers but not maxpacket-aligned, causing usb_submit_urb
to return -EINVAL before the URB reaches the host controller.

xHCI-class controllers have no such DMA alignment requirement, so set
no_sg_constraint on the SS bus to match.  Leave the HS bus unchanged:
EHCI-class controllers do have this requirement in hardware, and
dummy_hcd's HS roothub is meant to emulate that behaviour, so the
constraint should stay enforced there.

Suggested-by: Alan Stern <stern@rowland.harvard.edu>
Assisted-by: OpenCode:claude-sonnet-5
Signed-off-by: Igor Skalkin <igor.skalkin@oss.qualcomm.com>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Link: https://patch.msgid.link/20260812081910.6821-3-igor.skalkin@oss.qualcomm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>