rtc: sunxi: Replace deprecated rtc_tm_to_time()

sunxi_rtc_setalarm() 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()

Also remove the unnecessary initial zeroing of some
local variables in sunxi_rtc_setalarm().

Cc: Carlo Caione <carlo.caione@gmail.com>
Signed-off-by: Xunlei Pang <pang.xunlei@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>