rtc: Cleanup for UIE timer/polling emulation support

hulk inclusion
category: bugfix
bugzilla: 186781, https://gitee.com/openeuler/kernel/issues/I61CEW


CVE: NA

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Split out a function that does not acquire ops_lock from
rtc_update_irq_enable, in preparation for fixing RTC_RD_TIME
and RTC_UIE_ON race problem.

Signed-off-by: Yu Liao <liaoyu15@huawei.com>
Reviewed-by: Wei Li <liwei391@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>