USB: serial: add missing atomic includes

Add the missing atomic.h include to the two driver that use it but did
not include it directly.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/io_edgeport.c

@@ -25,6 +25,7 @@
  *
  */
 
+#include <linux/atomic.h>
 #include <linux/kernel.h>
 #include <linux/jiffies.h>
 #include <linux/errno.h>

drivers/usb/serial/mos7720.c

@@ -17,6 +17,8 @@
  *	Copyright (C) 2000 Inside Out Networks, All rights reserved.
  *	Copyright (C) 2001-2002 Greg Kroah-Hartman <greg@kroah.com>
  */
+
+#include <linux/atomic.h>
 #include <linux/kernel.h>
 #include <linux/errno.h>
 #include <linux/slab.h>