libata: unbreak TPM filtering by reorganizing ata_scsi_pass_thru()

ata_scsi_pass_thru() was checking for input sanity and disallowed
commands while initializaing qc from scmd.  TPM filtering was added
right after protocol check at which point tf wasn't initialized
properly.  This means that TPM filtering has never really worked.

This patch fixes the bug by reorganizing ata_scsi_pass_thru() such
that qc is fully initialized before checking for invalid conditions
which is way less error prone.

Discovered while Thilo-Alexander Ginkel was trying debug patches for
bko#13416.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Thilo-Alexander Ginkel <thilo@ginkel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>