mpr(4): Fix unmatched devq release.

Before this change devq was frozen only if some command was sent to
the target after reset started, but release was called always. This
change freezes the devq immediately, leaving mprsas_action_scsiio()
check only to cover race condition due to different lock devq use.

This should also avoid unnecessary requeue of the commands, creating
additional log noise and confusing some broken apps.

MFC after: 2 weeks
Sponsored by: iXsystems, Inc.