drivers/isdn/hardware/mISDN: change type of hfc_jiffies to unsigned long

Jiffies are unsigned long, make sure we fit in jiffies store variable
on archs with bits per long > 32.

Patch suggested by Jiri Slaby.

Signed-off-by: Hannes Eder <hannes@hanneseder.net>
Signed-off-by: David S. Miller <davem@davemloft.net>