[PATCH] libata: improve ata_bus_probe()

Improve ata_bus_probe() such that configuration failures are handled
better.  Each device is given ATA_PROBE_MAX_TRIES chances, but any
non-transient error (revalidation failure with -ENODEV, configuration
failure with -EINVAL...) disables the device directly.  Any IO error
results in SATA PHY speed down and ata_set_mode() failure lowers
transfer mode.  The last try always puts a device into PIO-0.

After each failure, the whole port is reset to make sure that the
controller and all the devices are in a known and stable state.  The
reset also applies SATA SPD configuration if necessary.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>