pms(4): Do not return CAM_REQ_CMP on errors.

It is a direct request for data corruptions, one report of which we
have received. I am very surprised that only one.

MFC after: 1 week
Sponsored by: iXsystems, Inc.

(cherry picked from commit 8434a65ce49bd6bc6779f0e57b0ce0f4bc46f48e)