rtc: Fix some bugs that allowed accumulating time drift in suspend/resume

The current code checks if abs(delta_delta.tv_sec) is greater or
equal to two before it discards the old delta value, but this can
trigger at close to -1 seconds since -1.000000001 seconds is stored
as tv_sec -2 and tv_nsec 999999999 in a normalized timespec.

rtc_resume had an early return check if the rtc value had not changed
since rtc_suspend. This effectivly stops time for the duration of the
short sleep. Check if sleep_time is positive after all the adjustments
have been applied instead since this allows the old_system adjustment
in rtc_suspend to have an effect even for short sleep cycles.

CC: stable@kernel.org
Signed-off-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>