ata: libata-core: Remove ata_exec_internal_sg()

ata_exec_internal() is the only caller of ata_exec_internal_sg() and
always calls this function with a single element scattergather list.
Remove ata_exec_internal_sg() and code it directly in
ata_exec_internal(), simplifying a little the sgl handling for the
command.

While at it, change the function signature to use the proper enum
dma_data_direction type for the dma_dir argument, cleanup comments
(capitalization and remove useless comments) and change the variable
auto_timeout type to a boolean.

No functional change.

Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Niklas Cassel <cassel@kernel.org>