rtc: Optimize calculations in rtc_time64_to_tm()

rtc: Optimize calculations in rtc_time64_to_tm()

Recently (in commit 7df4cfef8b35 ("rtc: Make rtc_time64_to_tm() support
dates before 1970")) the function rtc_time64_to_tm() was repaired for
times before 1970. This introduced two if blocks. Cassio Neri pointed
out that to be not neccessary and suggested an adaption that allows to
drop the two branch points again.

This is implemented here.

Also adapt the reference to the theoretical paper to link to the final
published article instead of the preprint on Cassio's request.

Suggested-by: Cassio Neri <cassio.neri@gmail.com>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Link: https://lore.kernel.org/r/20250613142405.253420-2-u.kleine-koenig@baylibre.com
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>