TxMapping controls the mapping of the Internal Service Class
(ISC) to the ToS/DS byte. It consists of a list of
(pattern, mask, ISC, DP) tuples stored in the same format as
wfIpTosTemplateCfgRxMapping. For a packet with a given ISC
and DP, the bits selected by 'mask' in the the packet's
ToS byte will be modified to contain 'pattern'.