audit: re-structure audit field valid checks

Multiple checks were being done in one switch case statement that
started to cause some redundancies and awkward exceptions.  Separate the
valid field and op check from the select valid values checks.

Enforce the elimination of meaningless bitwise and greater/lessthan
checks on string fields and other fields with unrelated scalar values.

Please see the github issue
https://github.com/linux-audit/audit-kernel/issues/73



Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>