Input: fix input.h kernel-doc warning

Fix kernel-doc warning in input.h:

Warning(include/linux/input.h:140): No description found for parameter 'len'
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

include/linux/input.h

@@ -116,6 +116,7 @@ struct input_keymap_entry {
 
 /**
  * EVIOCGMTSLOTS(len) - get MT slot values
+ * @len: size of the data buffer in bytes
  *
  * The ioctl buffer argument should be binary equivalent to
  *