Support stopping and restarting the AHCI command list via toggling
PxCMD.ST from '1' to '0' and back.  This allows the driver a chance to
recover if for instance a timeout occurred due to activity on the
host.

Reviewed by:	grehan


git-svn-id: svn+ssh://svn.freebsd.org/base/head@273212 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f