Cap the number of streams supported to two for now.

I haven't yet reviewed the Intel driver(s) in more depth to see if
there are 1x1 NICs that report they support 2 transmit/receive chains..
if so then we'll have to update this.

Tested:

• Intel 4965, which is a 2x2 device with 3 RX and 2 TX chains.

PR: kern/181132