iwm: port iwm_enable_txq() from OpenBSD; track ring->tail

[adrian]

Summary

• Add in some of the bits to track ring->tail as in OpenBSD. It's not yet complete; I still need to port iwm_txq_advance() and update the TX completion path to use it.

• Port over iwm_enable_txq() from OpenBSD, which includes the setup bits needed for enabling TX aggregation queues.

Locally tested:

• 7260, STA mode

Obtained from: OpenBSD a6e25c727a2214219715b51ba6af2221d7e0d674