watchdog: pm8916_wdt: fix pretimeout registration flow

When an IRQ is present in the dts, the probe function shall fail if
the interrupt can not be registered.

The probe function shall also be retried if getting the irq is being
deferred.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>
Reviewed-by: Loic Poulain <loic.poulain@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>