rtc: Replace flush_scheduled_work() with flush_work().

Since "struct rtc_device" is per a device struct, I assume that clear_uie()
needs to wait for only one work associated with that device. Therefore,
wait for only that work using flush_work().

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/8d3a0f55-d861-ba93-0d25-b1172eaa8343@I-love.SAKURA.ne.jp