mISDN: Add watchdog functionality to hfcmulti driver

This patch was made by Titus Moldovan and provides IOCTL functions for enabling
and disabling the controller's built in watchdog. The use is optional.

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