ata: libata-eh: avoid unnecessary calls to ata_scsi_port_error_handler()

When handling SCSI command timeouts, if we had no actual command
timeouts (either because the command was a deferred qc or the completion
path won the race with ata_scsi_cmd_error_handler()), we do not need to
go through a port error handling, as there was in fact no errors at all.

Modify ata_scsi_cmd_error_handler() to return the number of commands
that timed out and use this return value in ata_scsi_error() to call
ata_scsi_port_error_handler() only if we had command timeouts, or if
the port EH has already been scheduled due to failed commands.
Otherwise, simply call scsi_eh_flush_done_q() to finish the completed
commands without running the full port error handling.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Niklas Cassel <cassel@kernel.org>