Fix generic device classification bug for devices w/o L1-L3 support

Commit 52275e92 made a small change in generic device classification
based on sysObjectID for devices that don't report Layer1 - Layer3
capabilities, in order to skip generic assignment if a more specific
class had already been determined. However this change contained an
error causing generic classification to fail.
Nex fix explicitly checks if class isn't more specific than SNMP::Info
before attempting to classify based on sysObjectID.

Info.pm

@@ -1499,7 +1499,7 @@ sub device_type {
             if ( $desc =~ /HP\sVC\s/ );
 
         # Generic device classification based upon sysObjectID
-        if ( defined($id) and !defined $objtype ) {
+        if ( defined($id) and $objtype eq 'SNMP::Info') {
             if ( defined $l3sysoidmap{$id} ) {
                 $objtype = $l3sysoidmap{$id};
             } elsif ( defined $l2sysoidmap{$id}) {