qc used to be freed automatically on command completion.  However, as
a qc can carry information about its completion status, it can be
useful to its owner/issuer after command completion.  This patch makes
freeing qc responsibility of its owner.  This simplifies
ata_exec_internal() and makes command turn-around for atapi request
sensing less hackish.

This change was originally suggested by Jeff Garzik.
Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>