HID: compilation fix when DEBUG_DATA is defined

hid/hid-core.c references 'len' variable when DEBUG_DATA is defined,
but the actual name of the variable is 'size'. Fix it.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
diff --git a/drivers/hid/hid-core.c b/drivers/hid/hid-core.c
index 38db832..b8cf50f 100644
--- a/drivers/hid/hid-core.c
+++ b/drivers/hid/hid-core.c
@@ -951,7 +951,7 @@
 	}
 
 #ifdef DEBUG_DATA
-	printk(KERN_DEBUG __FILE__ ": report (size %u) (%snumbered)\n", len, report_enum->numbered ? "" : "un");
+	printk(KERN_DEBUG __FILE__ ": report (size %u) (%snumbered)\n", size, report_enum->numbered ? "" : "un");
 #endif
 
 	n = 0;                          /* Normally report number is 0 */