KVM: PPC: BookE: Fix a sanity check

We use logical negate where bitwise negate was intended.  It means that
we never return -EINVAL here.

Fixes: ce11e48b ('KVM: PPC: E500: Add userspace debug stub support')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>