Turn off the code which waits for AHCI_PREG_CMD_CR to be set by the
HBA after ahci_default_port_start() sets AHCI_PREG_CMD_ST. The AHCI
spec. rev. 1.3 only requires the inverse to be true, i. e. that a
HBA clears AHCI_PREG_CMD_CR when AHCI_PREG_CMD_ST gets cleared by
software/driver. In fact, some HBAs will not raise AHCI_PREG_CMD_CR
as an immediate consequence of AHCI_PREG_CMD_ST being set. Actually
neither the FreeBSD, Linux nor NetBSD counterpart of ahci(4) has an
analogous check. Disabling that wait fixes "failed to start command
DMA on port N, disabling" bails during attach.
From Marius Strobl
ok deraadt@ jmatthew@
UnifiedSplitRaw
