TX packets, which can be either Ethernet or control packets, must be

TX packets, which can be either Ethernet or control packets, must be
sent in order.  Otherwise it is possible that the key is set before
we send out the EAPOL packet, or that packets are sent out before
the key is set.  Thus modify the SDIO backend to put both types into
the same internal TX queue, which will be sent asynchronously.

Discussed with bluhm@