rtcwake: fix sys vs. rtc time delta usage

rtcwake: fix sys vs. rtc time delta usage

srett commented:

 After some head scratching why relative wakeup using -s always works
 while -t didn't seem to work at all I discovered that the adjustment
 for time difference between sys and rtc is actually inverted when
 writing the alarm (code for -m show reading is fine), so if the RTC
 lags 10 minutes behind, the alarm will be shifted into the future by
 10 minutes, resulting in a wakeup that is 20 minutes late.

Addresses: https://github.com/karelzak/util-linux/issues/544
Signed-off-by: Karel Zak <kzak@redhat.com>