ata: libata: clarify when ata_eh_request_sense() will be called

ata: libata: clarify when ata_eh_request_sense() will be called

ata_eh_request_sense() returns early when flag ATA_QCFLAG_SENSE_VALID is
set. However, since the call to ata_eh_request_sense() is guarded by a
ATA_SENSE bit conditional, the logical conclusion for the reader is that
all checks are performed at the call site.

Highlight the fact that the sense data will not be fetched if flag
ATA_QCFLAG_SENSE_VALID is already set by adding an additional check to
the existing guarding conditional. No functional change.

Additionally, add a comment explaining that ata_eh_analyze_tf() will
only fetch the sense data if:
-It was a non-NCQ command that failed, or
-It was a NCQ command that failed, but the sense data was not included
 in the NCQ command error log (i.e. NCQ autosense is not supported).

Signed-off-by: Niklas Cassel <niklas.cassel@wdc.com>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>