[iwn] fill in the TX command rate info flags consistently.

The Linux driver sets the rate_n_flags regardless of whether it's being
sent using firmware rate control or local rate control. This includes
the antenna configuration.

Thanks to Kyle Evans <kevans91@ksu.edu> for pointing this out to me
and doing some investigation/testing on his end.

Tested:

• Intel 7260 STA, 2G and 5G networks