Mesh HWMP forwarding information: updating FI for transmitter.
Mesh HWMP forwarding information: updating FI for transmitter.

* Added hwmp_update_transmitter function that checks if the metric
  to the transmitter have improved. If old FI is invalid or metric
  is larger the FI to the transmitter is updated occurdingly.
  This is a recommendation from the 802.11 2012 standard, table 13-9;

Approved by:	adrian (mentor)

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