mISDN: Add X-Tensions USB ISDN TA XC-525

According to http://www.ip-phone-forum.de/showthread.php?t=225313

 this
HW works. Thanks to Lars Immisch for pointing to this thread.

Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>