shwdt: fix usage of mod_timer

This patch fixes the usage of mod_timer and makes the driver usable.
mod_timer must be called with an absolute timeout in jiffies, the old
implementation used a relative timeout thus the hardware watchdog was
never triggered.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>