iir: prevent negative offsets in ioctl

MFS of direct commit to stable/13 as this driver has been removed from
main in commit 399188a2c60c ("iir: Remove").

PR: 273328
Reported by: ChenHao Lu
Sponsored by: The FreeBSD Foundation

(cherry picked from commit b5a5a06fc012d27c6937776bff8469ea465c3873)