usb: core: add urb->sgt parameter description

usb: core: add urb->sgt parameter description

The parameter description of urb->sgt is lost, this will add it for
completeness.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/all/20250711182803.1d548467@canb.auug.org.au/
Signed-off-by: Xu Yang <xu.yang_2@nxp.com>
Fixes: 488e6eaab88c ("usb: core: add dma-noncoherent buffer alloc and free API")
Link: https://lore.kernel.org/r/20250721104417.3442530-1-xu.yang_2@nxp.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>