mi: Build fix: mieqProcessDeviceEvent returns void

mieq.c:520:9: error: void function 'mieqProcessDeviceEvent' should not return a value [-Wreturn-type,Semantic Issue]
        return 0;
        ^      ~
1 error generated.

Regression-from: 9fb08310b51b46736f3ca8dbc04efdf502420403
Found-by: Tinderbox

Signed-off-by: Jeremy Huddleston Sequoia <jeremyhu@apple.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>