clocksource/drivers/keystone: Fix bad NO_IRQ usage

The current code assumes the 'irq_of_parse_and_map' will return NO_IRQ in case
of failure. Unfortunately, the NO_IRQ is not consistent across the different
architectures and we must not rely on it.

NO_IRQ is equal to '-1' on ARM and 'irq_of_parse_and_map' returns '0' in case
of an error. Hence, the latter won't be detected and will lead to a crash.

Fix this by just checking 'irq' is different from zero.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>