watchdog: shwdt: fix usage of mod_timer

Fix the usage of mod_timer() and make 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>
Signed-off-by: Wim Van sebroeck <wim@iguana.be>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: stable <stable@kernel.org>