sata_sil24: Use memory barriers before issuing commands

The data in the cmd_block buffers may reach the main memory after the
writel() to the device ports. This patch introduces two calls to wmb()
to ensure the relative ordering.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Tested-by: Colin Tuckley <colin.tuckley@arm.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>