MFC r252203:
Add test for SATA registers writability and skip using them if it failed.

There are some systems reported, where PCI BAR(5), used for SATA registers
access, is present, but not functional.  Attempt to use it brakes devices
detection logic.  Try to detect those cases on attach by setting and testing
some bits in SControl register.  If bits are unsettable, fallback to legacy
ATA without hot-plug detection, speed control/reporting, etc.