ata: libata: ata_{sff|std}_prereset() always return 0

ata_std_prereset() always returns 0, hence the check in ata_sff_prereset()
is pointless and thus it also can return only 0 (however, we cannot change
the prototypes of ata_{sff|std}_prereset() as they implement the driver's
prereset() method).

Found by Linux Verification Center (linuxtesting.org) with the SVACE static
analysis tool.

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