Riemann: fix "Decode as" feature

the UDP port value check seems useless when the port is configured in the preferences, and prevents "Decode as" to work as expected

Change-Id: I5a059d05a9e980c2c60d8f84deae1b5bd9d466d3
Reviewed-on: https://code.wireshark.org/review/5727
Reviewed-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>