gicv3: Stop setting the esize field

The GITS_BASER esize field is read-only, there is no need to change it.

Reviewed by: gallatin, imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D41552

(cherry picked from commit 2f11b2abfc185d3d4f45e6a3dae270b859cdd388)