ktls_ocf: Fix a few places to not hardcode the GMAC hash length.

This is not a functional change as the Poly1305 hash is the same
length as the GMAC hash length.

Reviewed by: gallatin, markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D30137

(cherry picked from commit 4a92afae7fcbb0a8453712dfec5de086aaf5cba4)