ata: libata-sff: refactor ata_sff_set_devctl()

Commit 41dec29b

 ("libata: introduce sff_set_devctl() method") left some
clumsy checks surrounding calls to ata_sff_set_devctl() which Jeff Garzik
suggested to factor out...  and I never followed up. :-(

At last, refactor ata_sff_set_devctl() to include the repetitive checks and
return a 'bool' result indicating if the device control register exists or
not.

While at it, further update the 'kernel-doc' comment -- the device control
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>