m68knommu: mask of vector bits in exception word properly

The vector field of the processors exception frame actually contains
both the vector exception number and fault status field bits.
The exception processing code was not correctly masking out the
fault status field bits before switching on the vector number.
The default case was catching the bad check, but we are reporting
the wrong kind of exception in some cases.

Reported-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>