ata: libata-eh: Cleanup ata_scsi_cmd_error_handler()

If ap->ops->error_handler is NULL just return. This patch also
fixes some comment style issue.

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