rtc: rx8010: Fix for incorrect return value

The err variable is not being reset after a successful read. Explicitly
return 0 at the end of function call to account for all return paths.

Reported-by: Jens-Peter Oswald <oswald@lre.de>
Signed-off-by: Akshay Bhat <akshay.bhat@timesys.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>