rtl8xxxu: Pass RX rate to rx_parse_phystats and enable phystats for rtl8723bu

rtl8xxxu_rx_parse_phystats() only needs the RX rate to determine
whether to handle the stats as CCK or not. Parsing in the rate rather
than the rx descriptor elimantes the need to handle multiple rx
descriptor formats in the function.

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