rtc: Replace simple_strtoul by kstrtoul

The simple_strtoul function is obsolete.
This patch replace it by kstrtoul.

Since kstrtoul is more strict, it permits to filter some invalid input that
simple_strtoul accept. For example:
echo '1022xxx' > /sys/devices/pnp0/00:03/rtc/rtc0/max_user_freq
cat /sys/devices/pnp0/00:03/rtc/rtc0/max_user_freq
1022

Signed-off-by: LABBE Corentin <clabbe.montjoie@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>