USB: ftdi driver - add support for optical probe device

Added support for the Elster Unicom III Optical Probe.
The device ID has already been added to the usb.ids file.

Cc: stable <stable@kernel.org>
Signed-off-by: Ed Beroset <beroset@mindspring.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/drivers/usb/serial/ftdi_sio.h b/drivers/usb/serial/ftdi_sio.h
index abd632c..f6053da 100644
--- a/drivers/usb/serial/ftdi_sio.h
+++ b/drivers/usb/serial/ftdi_sio.h
@@ -536,6 +536,8 @@
 #define OLIMEX_VID			0x15BA
 #define OLIMEX_ARM_USB_OCD_PID		0x0003
 
+/* www.elsterelectricity.com Elster Unicom III Optical Probe */
+#define FTDI_ELSTER_UNICOM_PID		0xE700 /* Product Id */
 
 /*
  * The Mobility Lab (TML)