intel_scu_ipcutil: fix major device number handling

We need to save major device number to be able to use it for
unregister_chrdev() in ipc_module_exit().

ipc_module_init() must return 0 on success, not major device number.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>