ide: fix crash at boot with siimage driver

Some change to the IDE layer are causing the siimage driver to crash
at boot with a NULL dereference. This is due to the sil_dma_ops not
containing all the necessary pointers. I suppose it used to just
"override" the defaults while now, it needs to contain everything.

[bart: while at it: sil_dma_ops should be const now (pointed out by Sergei)]

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>,
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>