ata: libata-scsi: Do not overwrite valid sense data when CK_COND=1

Current ata_gen_passthru_sense() code performs two actions:
1. Generates sense data based on the ATA 'status' and ATA 'error' fields.
2. Populates "ATA Status Return sense data descriptor" / "Fixed format
   sense data" with ATA taskfile fields.

The problem is that #1 generates sense data even when a valid sense data
is already present (ATA_QCFLAG_SENSE_VALID is set). Factoring out #2 into
a separate function allows us to generate sense data only when there is
no valid sense data (ATA_QCFLAG_SENSE_VALID is not set).

As a bonus, we can now delete a FIXME comment in atapi_qc_complete()
which states that we don't want to translate taskfile registers into
sense descriptors for ATAPI.

Additionally, always set SAM_STAT_CHECK_CONDITION when CK_COND=1 because
SAT specification mandates that SATL shall return CHECK CONDITION if
the CK_COND bit is set.

The ATA PASS-THROUGH handling logic in ata_scsi_qc_complete() is hard
to read/understand. Improve the readability of the code by moving checks
into self-explanatory boolean variables.

Cc: [email protected] # 4.19+
Co-developed-by: Niklas Cassel <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Reviewed-by: Damien Le Moal <[email protected]>
Reviewed-by: Niklas Cassel <[email protected]>
Signed-off-by: Igor Pylypiv <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Niklas Cassel <[email protected]>

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