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

drivers/usb/gadget/legacy/printer.c

@@ -50,6 +50,7 @@ MODULE_PARM_DESC(iPNPstring, "MFG:linux;MDL:g_printer;CLS:PRINTER;SN:1;");
 /* Number of requests to allocate per endpoint, not used for ep0. */
 static unsigned qlen = 10;
 module_param(qlen, uint, S_IRUGO|S_IWUSR);
+MODULE_PARM_DESC(qlen, "The number of 8k buffers to use per endpoint");
 
 #define QLEN	qlen