Follow up from r324201.mlx4: use enum constants instead of const vars for case exprs
I struggled to find a formulation that was not terribly ugly. Please feel free to make a suggestion or take this over.Follow up from r324201 to fix compilation with gcc, which complains
about non-ICE case expressions.
Reviewed by: hselasky
Differential Revision: https://reviews.freebsd.org/D12675