mfd: cros_ec: stop calling ->cmd_xfer() directly

Instead of having users of the ChromeOS EC call the interface-specific
cmd_xfer() callback directly, introduce a central cros_ec_cmd_xfer()
to use instead.  This will allow us to put all the locking and retry
logic in one place instead of duplicating it across the different
drivers.

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/i2c/busses/i2c-cros-ec-tunnel.c

@@ -227,7 +227,7 @@ static int ec_i2c_xfer(struct i2c_adapter *adap, struct i2c_msg i2c_msgs[],
 	msg.indata = response;
 	msg.insize = response_len;
 
-	result = bus->ec->cmd_xfer(bus->ec, &msg);
+	result = cros_ec_cmd_xfer(bus->ec, &msg);
 	if (result < 0)
 		goto exit;