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>
diff --git a/include/linux/input.h b/include/linux/input.h
index a816714..2740d08 100644
--- a/include/linux/input.h
+++ b/include/linux/input.h
@@ -116,6 +116,7 @@
 
 /**
  * EVIOCGMTSLOTS(len) - get MT slot values
+ * @len: size of the data buffer in bytes
  *
  * The ioctl buffer argument should be binary equivalent to
  *