netdevice pc300: Add the reason about PC300 BROKEN in Kconfig

When compile test my previous patch, I found PC300 driver was broken.
And there is no explanation about the broken.
Add the reason about why change this driver to broken in Kconfig.
Signed-off-by: NWang Chen <wangchen@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/wan/Kconfig

@@ -207,6 +207,8 @@ config PC300
 	tristate "Cyclades-PC300 support (RS-232/V.35, X.21, T1/E1 boards)"
 	depends on HDLC && PCI && BROKEN
 	---help---
+	  This driver is broken because of struct tty_driver change.
+
 	  Driver for the Cyclades-PC300 synchronous communication boards.
 
 	  These boards provide synchronous serial interfaces to your