sata_mv: revert SoC irq breakage

Revert most of commit 6be96ac1d5e4d913e1f48299db083ada5321803b2,
originally from Lennert Buijtenheck (Marvell) and Saeed Bishara (Marvell),
since that commit causes sata_mv to oops at startup on SOC "Kirkwood".

The SOC variants do not have the hpriv->irq_{cause,mask}_ofs registers,
so don't try to write to them!

This patch should also be considered for -stable.

Reported-by: Maxime Bizon <mbizon@freebox.fr>
Signed-off-by: Mark Lord <mlord@pobox.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>