iichid: Soften "Interrupt setup failed" message

Users may interpret the message as a possible hardware error, but the
issue is in fact unimplemented functionality. Reword the message to
avoid implying it is an error.

Reviewed by: wulf
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D46092

(cherry picked from commit 0ac6cc3fd7473a7a8596fa73322a67fc1d78df53)