LinuxKPI: 802.11: inialize tx queue params

When adding a VAP (vif) initialize its tx queue parameters calling
mo_conf_tx().
I could not spot net80211 providing some of the values needed before
having a node so currrently we use hard-coded values with a comment
with a reference on how to properly calculate the values in the future
(e.g., in case of 11b or other cases).

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