usb: dwc3: Document nostream_work

usb: dwc3: Document nostream_work

Add missing description to the nostream_work of dwc3_ep. The work is
used by bulk multi-stream endpoints for a NoStream event to reinitiate
the stream if needed.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Closes: https://lore.kernel.org/linux-next/20250120182219.30dcb3c6@canb.auug.org.au/
Signed-off-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/7cdeaa346d24907712aac533c1c5f90a03151189.1737500936.git.Thinh.Nguyen@synopsys.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>