rtl8xxxu: Enable aggregation for rtl8723au

Implement rtl8xxxu_gen1_init_aggregation(). Aggregation should be the
same for all gen1 parts. We may want to allow for tuning parameters in
the fileopes struct. For now this is based allocating 16KB RX buffers,
leaving 16000 bytes for actual packets, and the rest for the skb
overhead.

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