rtl8xxxu: Add support for aggregated RX packets on gen1 parts

This implements support for demuxing aggregated RX packets on gen1
devices, using the rxdesc16 format.

So far this has only been tested with rtl8723au devices.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>