kernel-doc: fix new warning in usb.h

Fix new kernel-doc warning:

Warning(include/linux/usb.h:1251): No description found for parameter 'num_mapped_sgs'
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

include/linux/usb.h

@@ -1073,6 +1073,7 @@ typedef void (*usb_complete_t)(struct urb *);
  *	which the host controller driver should use in preference to the
  *	transfer_buffer.
  * @sg: scatter gather buffer list
+ * @num_mapped_sgs: (internal) number of mapped sg entries
  * @num_sgs: number of entries in the sg list
  * @transfer_buffer_length: How big is transfer_buffer.  The transfer may
  *	be broken up into chunks according to the current maximum packet