watchdog: mtx1-wdt: fix build failure

Commit 72b6e8a8 (watchdog: mtx1-wdt: request gpio before using it) was
backported from upstream. The patch is using a gpiolib call which is only
available in kernel 2.6.34+. Fix build by using the "old" gpiolib API
instead.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>