parisc: mask out enable and reserved bits from sba imask

When using kexec the SBA IOMMU IBASE might still have the RE
bit set. This triggers a WARN_ON when trying to write back the
IBASE register later, and it also makes some mask calculations fail.

Cc: <stable@vger.kernel.org>
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Helge Deller <deller@gmx.de>