rtc: isl1208: do not advertise update interrupt feature if no interrupt specified

If an ISL1208 device does not have an interrupt line routed, the feature
shouldn't be advertised (it is by default in rtc core) or it'll confuse
userspace requesting that feature (such as hwclock from util-linux).

Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220523145320.123713-1-foss+kernel@0leil.net