sfxge(4): zero should be used as RxQ label in SW event

The buggy code was using the rxq index but should use the evq label
associated with the rxq. It was missed in r298735.

Reviewed by: gnn
Sponsored by: Solarflare Communications, Inc.
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D6661