rtc: don't use flush_scheduled_work()

flush_scheduled_work() is deprecated and scheduled to be removed.  On
removal, directly cancel the work, and flush the uie_task in
rtc-dev.c::clear_uie().

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: rtc-linux@googlegroups.com