This patch is to set register to allow the mbox enter sleep status
if it does not have tx traffic and wakeup it if tx traffic arrive.
After mbox enter sleep status, the soc will enter sleep status by
firmware, this will save power. The power consume drops from about
90mW to about 10mW with this patch.

This patch only effect sdio chip.

Tested with QCA6174 SDIO with firmware WLAN.RMH.4.4.1-00029.
Signed-off-by: Wen Gong <wgong@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1587037859-28873-3-git-send-email-kvalo@codeaurora.org