we may set a TFTP port via Decode As
take this setting into account when we try to detect the first packet of
a tftp transfer

Change-Id: Ia80f9b5f994b02ecaaa063308b1a87d00fe0dafe
Reviewed-on: https://code.wireshark.org/review/3450
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>