rtwn: add forcerate flag to TX descriptor setup

When doing firmware rate control there will be situations where
the rate being passed in needs to actually override the rate
control selection. So add a flag to the descriptor setup path
to indicate that indeed this particular rate should be forced,
rather than rely on the firmware rate control.

This is currently a no-op as firmware rate control isn't working
in-tree, but it is working for me locally with other changes.
Without this, there's no way to force low rates for management,
DHCP traffic, and to allow fixed rate via "ifconfig wlanX ucastrate Y"
to function.

Locally tested:

• RTL8192CU, STA mode (firmware and driver/net80211 rate control)

Differential Revision: https://reviews.freebsd.org/D48100
Reviewed by: bz, gavin