rtc: vt8500: Use define instead of hardcoded value for status bit

Fixes the vt8500 driver to use a define value for the isr
is-alarm bit.

Cc: Alexey Charkov <alchark@gmail.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>