rtc: mt6397: fix build on some 32bits platforms

On some !ARM 32bits platforms, the following compilation error happens
because of the division on a 64bits value in mtk_rtc_read_time():

drivers/built-in.o: In function `mtk_rtc_read_time':
rtc-mt6397.c:(.text+0x265d13f): undefined reference to `__divdi3'
rtc-mt6397.c:(.text+0x265d150): undefined reference to `__moddi3'

Use div_s64() as done in rtc_time64_to_tm() to solve that.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Acked-by: Eddie Huang <eddie.huang@mediatek.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>