rtc: pm8xxx: Return -ENODEV if set_time disallowed

Having !allow_set_time is equivalent to non-implemented set_time
function, which is normally represented with -ENODEV error in RTC
subsystem.

Today we are returning -EACCES error code, which is not considered
by RTC clients as a 'non implemented' feature, and which causes NTP
to retry hw clk sync (update_rtc) indefinitely.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1645090578-20734-1-git-send-email-loic.poulain@linaro.org