ata: ahci: move marking of external port earlier

Move the marking of an external port earlier in the call chain.
This is needed for further cleanups.
No functional change intended.

Tested-by: Damien Le Moal <dlemoal@kernel.org>
Tested-by: Jian-Hong Pan <jhp@endlessos.org>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Niklas Cassel <cassel@kernel.org>