rtl8xxxu: Add a couple of new register definitions

This adds some additional register definitions for 8723bu, as well as
a bit define for USB RXDMA aggregation in REG_RXDMA_AGG_PG_TH.

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