ahci/xgene: Remove logic to set 64-bit DMA mask

Instead of doing the check here, this should be handled
in the common AHCI platform code.

Signed-off-by: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Suggested-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Loc Ho <lho@apm.com>
Cc: Tuan Phan <tphan@apm.com>
Cc: Suman Triphati <stripathi@apm.com>
Signed-off-by: Tejun Heo <tj@kernel.org>