m68knommu: correctly use trap_init

Currently trap_init() is an empty function for m68knommu. Instead
the vectors are being setup as part of the IRQ initialization.
This is inconsistent with m68k and other architectures.

Change the local init_vectors() to be trap_init(), and init the
vectors at the correct time during startup. This will help merge of
m68k and m68knommu trap code in the furture.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>