rtc: pl031: remove useless invalid alarm handling

The core will never pass an invalid alarm to .set_alarm, it is not
necessary to check for its validity.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20200306005809.38530-1-alexandre.belloni@bootlin.com


Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>