usb: gadget: Add description for module parameter

The description for "qlen" is missing, and there is a description
for this parameter in "Documentation/usb/gadget_printer.rst"

Signed-off-by: Wei Ming Chen <jj251510319013@gmail.com>
Link: https://lore.kernel.org/r/20210821142647.2904-1-jj251510319013@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>