rtc: ab8500: remove useless check

rtc_time_to_tm always rturns a valid tm, there is no need to validate it.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>