watchdog: lantiq: fix watchdogs timeout handling

The enable function was using the global timeout variable for local operations.
This resulted in the value of the global variable being corrupted, thus
breaking the code.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Thomas Langer <thomas.langer@lantiq.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: linux-watchdog@vger.kernel.org
Cc: linux-mips@linux-mips.org