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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/gadget/legacy/printer.c

···
 /* 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