tty: n_smux: Add retry queue size flow control

If the receive retry queue size gets too large, then automatically
enable flow control to prevent having to drop buffers.

Change-Id: If27599015b362ce013e177ee350e026933390d72
Signed-off-by: Eric Holmberg <eholmber@codeaurora.org>
diff --git a/include/linux/smux.h b/include/linux/smux.h
index 308f969..24a6371 100644
--- a/include/linux/smux.h
+++ b/include/linux/smux.h
@@ -77,6 +77,8 @@
 	SMUX_TIOCM_UPDATE,
 	SMUX_LOW_WM_HIT,      /* @metadata is NULL */
 	SMUX_HIGH_WM_HIT,     /* @metadata is NULL */
+	SMUX_RX_RETRY_HIGH_WM_HIT,  /* @metadata is NULL */
+	SMUX_RX_RETRY_LOW_WM_HIT,   /* @metadata is NULL */
 };
 
 /**
@@ -86,6 +88,7 @@
 	SMUX_CH_OPTION_LOCAL_LOOPBACK = 1 << 0,
 	SMUX_CH_OPTION_REMOTE_LOOPBACK = 1 << 1,
 	SMUX_CH_OPTION_REMOTE_TX_STOP = 1 << 2,
+	SMUX_CH_OPTION_AUTO_REMOTE_TX_STOP = 1 << 3,
 };
 
 /**