rtl8xxxu: TX RTS rate is word 4 for 8723a

Correct the setting of TX RTS for 8723a generation chips. In addition
update documentation to match that this is part of data word 4, note
data word 5.

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