Updates to snmp.php to add the following devices

Cisco 2960 - OID 9.1.1316 (#933)
Cisco Me3400 - OID 9.1.1007 (#939)
HP 1800-24 J9028B - OID 11.2.3.7.11.81 (#941)
Cisco 2921 - OID 9.1.1044

Also moved check for known switches so as to not do snmp queries if switch not known