scsi: libsas: Add dev_parent_is_expander() helper

scsi: libsas: Add dev_parent_is_expander() helper

Many libsas drivers check if the parent of the device is an expander.
Create a helper that the libsas drivers will use in follow up commits.

Suggested-by: Damien Le Moal <dlemoal@kernel.org>
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Link: https://lore.kernel.org/r/20250814173215.1765055-15-cassel@kernel.org
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>