ahci: Workaround HW bug for SB600/700 SATA controller PMP support

There is one bug in ATI SATA PMP of SB600 and SB700 old revision, which leads
to soft reset failure. This patch can fix the bug.

Signed-off-by: Shane Huang <shane.huang@amd.com>
Acked-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>