- do not start a port before the BSY and DRQ flags have been cleared;
as such, poll on device status rather than signature availability.
- change "ahci_sig_timeout" to "ahci_device_timeout" variable setting
accordingly; this variable determines the polling duration before a
newly attached device is given up on, and uses 30 seconds as default;
- use port connect changes (PCS/X/DET->1h) to kick off polling for an
attached device; detachment is still detected by means of PhyRdy
status changes (PRCS/N/DET<->3h).
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