sh: Make is_valid_bugaddr() more intelligent on nommu.

Currently is_valid_bugaddr() is true for anything >= PAGE_OFFSET, which
happens to be 0 on nommu configurations. Make this a bit smarter by just
reading in the opcode and comparing it against the trap type that we
already know. Follows the logic from avr32.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>