ata: libata-sff: refactor ata_sff_altstatus()

ata: libata-sff: refactor ata_sff_altstatus()

The driver's calls to ata_sff_altstatus() are mostly surrounded by some
clumsy checks.  Refactor ata_sff_altstatus() to include the repetitive
checks and return a 'bool' result indicating if the alternate
status register exists or not.

While at it, further update the 'kernel-doc' comment -- the alternate
status register has never been a part of the taskfile, despite what
Jeff and co. think! :-)

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