ahci_xgene: Fix the link down in first attempt for the APM X-Gene SoC AHCI SATA host controller driver.

Due to HW errata the APM X-Gene AHCI SATA host controller reports link
down even if the device presence is detected. This issue is due to speed
negotiation failure. This patch implements the algorithm to retry the
COMRESET if PxSTAT register reports device presence detected but
PHY communication not established. The maximum retry attempts are 3.

This patch also fixes the code to match the algorithm for the printing
a warning message if the disparity error still exists after link up.
Signed-off-by: Loc Ho <lho@apm.com>
Signed-off-by: Suman Tripathi <stripathi@apm.com>
Signed-off-by: Tejun Heo <tj@kernel.org>