rtc: fsl-ftm-alarm: avoid struct rtc_time conversions

Directly call ktime_get_real_seconds instead of converting the result to a
struct rtc_time and then back to a time64_t.

Link: https://lore.kernel.org/r/20191016201223.30568-4-alexandre.belloni@bootlin.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>