opcodes: blackfin: decode insns with invalid register as illegal

opcodes: blackfin: decode insns with invalid register as illegal

Sometimes the encoding in the opcode is a 4 bit field which defines a
register number.  However, register numbers are only 0-7, so make sure
we call illegal for when the opcode register number is greater than 8.

Signed-off-by: Robin Getz <robin.getz@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>