ata: libata-scsi: use *switch* statements to check SCSI command codes

Replace strings of the *if* statements checking the SCSI command code
with the *switch* statements that fit better here...

Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Damien Le Moal <damien.lemoal@opensource.wdc.com>