MFC r220657:

MFC r220657:
Some changes around hot-plug and interface power-management:
 - use ATA_SE_EXCHANGED (SError.DIAG.X) bit to detect hot-plug events when
power-management enabled and ATA_SE_PHY_CHANGED (SError.DIAG.N) can't be
trusted;
 - on controllers supporting staggered spin-up (SS) put unused channels
into Listen state instead of Off. It should still save some power, but
allow plug-in events to be detected;
 - on controllers supporting cold presence detection (CPD), when power
management enabled, use CPD events to detect hot-plug in addition to PHY
events.

git-svn-id: svn://svn.freebsd.org/base/stable/8@220868 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f