timesyncd: clear ADJ_MAXERROR to keep STA_UNSYNC cleared after jump adjust (#4626)

NTP use jump adjust if system has incorrect time read from RTC during boot.
It is desireble to update RTC time as soon as NTP set correct system time.
Sometimes kernel failed to update RTC due to STA_UNSYNC get set before RTC
update finised. In that case RTC time wouldn't be updated within long time.
The commit makes RTC updates stable.

When NTP do jump time adjust using ADJ_SETOFFSET it clears STA_UNSYNC flag.
If don't clear ADJ_MAXERROR, STA_UNSYNC will be set again by kernel within
1 second (by second_overflow() function). STA_UNSYNC flag prevent RTC updates
in kernel. Sometimes the kernel is able to update RTC withing 1 second,
but sometimes it falied.
(cherry picked from commit 5f36e3d3)