Fix an issue that crept in with the previous descriptor tidyup.

Fix an issue that crept in with the previous descriptor tidyup.

When forming aggregates, the last descriptor was now not being
correctly setup - instead, the "setuplasttxdesc" call was being
handed the first descriptor in the last subframe, rather than the
last descriptor in the last subframe.

This showed up as "bad series0 hwrate" messages, as the final
descriptor just didn't have any of the rate control information
squirreled away.

Tested:
	* AR9280 STA -> 11n AP, iperf TCP