staging: set msi_domain_ops as __ro_after_init

Marked msi_domain_ops structs as __ro_after_init when called only during init.
This protects the data structure from accidental corruption.

Suggested-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Jess Frazelle <me@jessfraz.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>