rtl8xxxu: Move skb delivery into parse_tx_desc() handler

This is another prepatory patch to be able to handle aggregated RX
packets.

In order to avoid adding a prototype, this also moves the
rtl8723bu_handle_c2h() function.

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