ata: libata-acpi: Fix function name and provide description for 'prev_gtf'

ata: libata-acpi: Fix function name and provide description for 'prev_gtf'

Fixes the following W=1 kernel build warning(s):

 drivers/ata/libata-acpi.c:493: warning: expecting prototype for ata_acpi_gtm_xfermode(). Prototype was for ata_acpi_gtm_xfermask() instead
 drivers/ata/libata-acpi.c:648: warning: Function parameter or member 'prev_gtf' not described in 'ata_acpi_run_tf'

Cc: Jens Axboe <axboe@kernel.dk>
Cc: linux-ide@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210318085150.3131936-15-lee.jones@linaro.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>