isdn: icn: Fix potentially wrong string handling

This warning was reported by cppcheck:
drivers/isdn/icn/icn.c:1641: error: Dangerous usage of 'rev' (strncpy doesn't always 0-terminate it)

If strncpy copied 20 bytes, the destination string rev was not terminated.
The patch adds one more byte to rev and makes sure that this byte is
always 0.

Cc: Karsten Keil <isdn@linux-pingi.de>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Tejun Heo <tj@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: netdev@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: David S. Miller <davem@davemloft.net>