ahci: rewrite timeout initialization code

ahci: rewrite timeout initialization code

- the "ahci_sig_timeout" variable now denotes the entire checking
  period, and the delay between checks has been hardcoded; what was
  previously "ahci_sig_checks" is now computed from those two;
- timeout values are no longer used for both millisecond and clock
  tick values, and better typed;
- the computation of timeouts is no longer off by half a second.