platform/x86: ISST: Fix wrong unregister type

The MMIO driver is not unregistering with the correct type with the ISST
common core during module removal. This should be unregistered with
ISST_IF_DEV_MMIO instead of ISST_IF_DEV_MBOX.

Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>