hwclock: use carefully synchronize_to_clock_tick() return codes

hwclock: use carefully synchronize_to_clock_tick() return codes

* It seems that

	rtc-isl1208 0-006f: chip found, driver version 0.3
	rtc-isl1208 0-006f: rtc core: registered rtc-isl1208 as rtc0
	rtc-isl1208 0-006f: rtc power failure detected, please set clock.

  causes that hardware clock returns persistent time and synchronization
  is impossible. The hwclock(8) has to ignore this problem and allows to
  set clock anyway.

* synchronize_to_clock_tick() shouldn't to print the "...got clock tick"
  debug message in case of failure.

Signed-off-by: Karel Zak <kzak@redhat.com>