some reason. Restore it to correct 16.
HTF did hpcsh work all this time with hd6446x_imask[_IPL_N] indexed
with IPLs 0..15 is a mistery... Discovered by accidentally triggering
hd64461 timer interrupt that is not enabled and should have been
masked.
_IPL_N used to be *also incorrect* 15 before Andy's change, which
makes one wonder if dumb luck is severly underappreciated.
490f3711c0