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>