mlx5: Fix mismatch in array bounds.

Reported by: GCC -Warray-parameter
Reviewed by: hselasky, imp, emaste (earlier version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D37549

(cherry picked from commit 47cc457bd6c16352b47d3fb1288f745482a082af)