Improvements to soundness in snmp.AirOS backend #29

Modification in interfaces_mtu in snmp.AirOS backend according to #29

Modification in get_interfaces() in snmp.AirOS backend according to #29

Modification in interfaces_state in snmp.AirOS backend according to #29

Modification in interfaces_speed in snmp.AirOS backend according to #29

Modification in interfaces_bytes in snmp.AirOS backend according to #29

Modification in interfaces_MAC in snmp.AirOS backend according to #29

Modification in interfaces_type in snmp.AirOS backend according to #29