ata: ahci: a hotplug capable port is an external port

A hotplug capable port is an external port, so mark it as such.

We even say this ourselves in libata-scsi.c:
/* set scsi removable (RMB) bit per ata bit, or if the
 * AHCI port says it's external (Hotplug-capable, eSATA).
 */

This also matches the terminology used in AHCI 1.3.1
(the keyword to search for is "externally accessible").

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>