ath10k: htc: rx trailer lookahead support

The RX trailer parsing is now capable of parsing lookahead reports.
A lookahead contains the first 4 bytes of the next HTC message
(that will be read in the next SDIO read operation).
Lookaheads are used by the SDIO/mbox HIF layer to determine if
the next message is part of a bundle, which endpoint it belongs
to and how long it is.

Signed-off-by: Erik Stromdahl <erik.stromdahl@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>