audit: rework AUDIT_TTY_SET to only grab spin_lock once

We can simplify the AUDIT_TTY_SET code to only grab the spin_lock one
time.  We need to determine if the new values are valid and if so, set
the new values at the same time we grab the old onces.  While we are
here get rid of 'res' and just use err.

Signed-off-by: Eric Paris <eparis@redhat.com>