ata: libata-core: improve parameter names for ata_dev_set_feature()

ata_dev_set_feature() is currently used for enabling/disabling any ATA
feature, e.g. SETFEATURES_SPINUP and SETFEATURE_SENSE_DATA, i.e. it is
not only used to enable/disable SATA specific features.

For most features, the enable/disable bit is specified in the subcommand
specific field "count".
It is only for the specific subcommands "Enable SATA feature" (0x10) and
"Disable SATA feature" (0x90) where the field "count" is used to specify
a feature instead of enable/disable. The parameter names for this
function are thus quite misleading.

Rename the parameter names to be more generic and in line with ACS-5,
and remove the references to "SATA FEATURES" in the kernel-doc.

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