libata: fix DMA to stack in reading devslp_timing parameters

Commit 803739d2 ("[libata] replace
sata_settings with devslp_timing"), which was also Cc: stable, used a
stack buffer to receive data from ata_read_log_page(), which triggers
the following warning:
 ahci 0000:00:1f.2: DMA-API: device driver maps memory fromstack [addr=ffff880140469948]

Fix this by using ap->sector_buf instead of a stack buffer.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Cc: stable@kernel.org
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>