[SCSI] libsas: disable scanning lun > 0 on ata devices

Currently mvsas and pm8001 have custom ->slave_alloc implementations to
achieve this.  Uplevel it for all libsas drivers as isci encounters problems
with atapi devices when scanning past lun0.

Just do what Darrick suggested [1], and limit the scan for ata devices.

[1] http://marc.info/?l=linux-scsi&m=116604101119861&w=2

Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>