rtc: isl1208: Replace deprecated rtc_tm_to_time()

isl1208_i2c_set_alarm() uses deprecated rtc_tm_to_time(),
which will overflow in year 2106 on 32-bit machines.

This patch solves this by:
 - Replacing rtc_tm_to_time() with rtc_tm_sub()

Cc: Herbert Valerio Riedel <hvr@gnu.org>
Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>