wifi: Operational rate set is assumed to hold all mandatory rates

In a note in wifi-phy.h, it is mentioned that the Operational rate set of some devices may not contain all the mandatory rates mentioned in the standard, whereas in the ns-3 implementation assumes that the two sets are identical.

   * A quick note is appropriate here (well, here is as good a place
   * as any I can find)...
   *
   * In the standard there is no text that explicitly precludes
   * production of a device that does not support some rates that are
   * mandatory (according to the standard) for PHYs that the device
   * happens to fully or partially support.
   *
   * This approach is taken by some devices which choose to only support,
   * for example, 6 and 9 Mbps ERP-OFDM rates for cost and power
   * consumption reasons (i.e., these devices don't need to be designed
   * for and waste current on the increased linearity requirement of
   * higher-order constellations when 6 and 9 Mbps more than meet their
   * data requirements). The wording of the standard allows such devices
   * to have an OperationalRateSet which includes 6 and 9 Mbps ERP-OFDM
   * rates, despite 12 and 24 Mbps being "mandatory" rates for the
   * ERP-OFDM PHY.
   *
   * Now this doesn't actually have any impact on code, yet. It is,
   * however, something that we need to keep in mind for the
   * future. Basically, the key point is that we can't be making
   * assumptions like "the Operational Rate Set will contain all the
   * mandatory rates".

This note has been written by Dean in yans-wifi-phy.h back in 2010 (cd6ccc18) and moved to wifi-phy.h by Sébastien in 1e6e44b3