platform/x86/intel/sdsi: Fix bug in multi packet reads

Fix bug that added an offset to the mailbox addr during multi-packet
reads. Did not affect current ABI since it doesn't support multi-packet
transactions.

Fixes: 2546c600

 ("platform/x86: Add Intel Software Defined Silicon driver")
Signed-off-by: David E. Box <david.e.box@linux.intel.com>
Link: https://lore.kernel.org/r/20220420155622.1763633-4-david.e.box@linux.intel.com
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>