ahci_xgene: Fix the watermark threshold for the APM X-Gene SATA host controller driver.

As per SATA IO specification, when Host sends HOLD, the device takes
about 20DW latency to reply to HOLDA. In some case, device doesn't
response to HOLDA over 20DW and causes FIFO goes into over flow
condition. Due to this condition, device enumerations fails with
those devices. This patch adjust the watermark FIFO by increasing
the FIFO depth from 0x16(default) to 0x30 to address this issue.

Signed-off-by: Loc Ho <lho@apm.com>
Signed-off-by: Suman Tripathi <stripathi@apm.com>
Signed-off-by: Tejun Heo <tj@kernel.org>