Follow-up to #6666, see #6666 (comment)

The EvolveB stencil at boundary nodes of nodal B components accesses ghost cells of 2 for cell-centered E. All Yee-like solvers (Yee, CKC, Cylindrical, Spherical) are affected.

Fix: change GetMaxGuardCell() to return IntVect{2,2,2}

This single fix also resolves the single-precision comm path issue, because ng_FieldSolver flows from GetMaxGuardCell() into both SP and non-SP paths.