watchdog: imx2_wdt: Fix the argument of watchdog_active()

Fix the following build warning by passing the expected argument type to
watchdog_active():

drivers/watchdog/imx2_wdt.c: In function 'imx2_wdt_suspend':
drivers/watchdog/imx2_wdt.c:340:2: warning: passing argument 1 of 'watchdog_active' from incompatible pointer type [enabled by default]
In file included from drivers/watchdog/imx2_wdt.c:38:0:
include/linux/watchdog.h:104:20: note: expected 'struct watchdog_device *' but argument is of type 'struct watchdog_device **'

Reported-by: Olof's autobuilder <build@lixom.net>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>