macro: relax CONST_MAX() type check a tiny bit

macro: relax CONST_MAX() type check a tiny bit

This checked for strict type compatibility so far, which mean CONST_MAX()
couldn't be used on two differently signed integers, even though
conceptually there's nothing wrong with allowing that here, as C
correctly picks the larger type in the ternary op.

hence, let's explicitly whitelist integer comparisons here, as long as
the signedness matches.