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).

Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)

(cherry picked from commit a6042e17c8999c4965bd4d762c4564b322f1ae7b)
(cherry picked from commit 68e1e25baf2e5576011ed532d5a30e70997d488c)