isdn: fix NUL (\0 or \x00) specification in string

In C one can either use '\0' or '\x00' (or '\000') to add a NUL byte to
a string. '\0x00' isn't part of these and will in fact result in a
single NUL followed by "x00". This fixes that.

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Reported-at: http://www.viva64.com/en/b/0299/


Signed-off-by: David S. Miller <davem@davemloft.net>