[PATCH] sata_sil24: implement NCQ support

Implement NCQ support.  Sil24 has 31 command slots and all of them are
used for NCQ command queueing.  libata guarantees that no other
command is in progress when it issues an internal command, so always
use tag 0 for internal commands.

Signed-off-by: Tejun Heo <htejun@gmail.com>