BUG/MINOR: mqtt: connect parser uses wrong bit field for TOPIC_ALIAS_MAXIMUM

BUG/MINOR: mqtt: connect parser uses wrong bit field for TOPIC_ALIAS_MAXIMUM

In mqtt_parse_connect(), the MQTT_PROP_TOPIC_ALIAS_MAXIMUM case was checking
and setting MQTT_FN_BIT_TOPIC_ALIAS instead of MQTT_FN_BIT_TOPIC_ALIAS_MAXIMUM.
This means duplicate detection for Topic-Alias-Maximum property was using the
wrong bitmask, and the actual Topic-Alias-Maximum bit was never set, making
duplicate detection ineffective for this property. The CONNACK parser already
had this correct.