Tidy up range check in misc.c

Tidy up range check in misc.c

For consistnecy with other range checks involving max_copper_layer, and to
drive home the fact we are checking for exceeding max_copper_layer + 2.
NB: The +2 relates to the bottom silk and top-silk layers, whose numbers
are used to denote the bottom and top layer groups respectively.

re-write:
  layer > max_copper_layer + 1

to become:
  layer >= max_copper_layer + 2