ata: ahci_brcmstb: disable DIPM support

The Broadcom STB SATA host controller does not support device
initiated power management. Disable support for this feature
so the driver never sends SETFEATURES commands to the device
to enable/disable DIPM.

Signed-off-by: Danesh Petigara <dpetigara@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>