ipmi_ssif: Fix inverted for the end of multi-part reads

As per Intelligent Platform Management Interface Specification v2.0 rev. 1.1,
section 12.5: SSIF Multi-part Read Transactions

Sponsored by: Ampere Computing LLC
Submitted by: Klara Inc.
Reviewed by: manu
Differential Revision: https://reviews.freebsd.org/D28749