spi: mux: repair mux usage

It is not valid to cache/short out selection of the mux.

mux_control_select() only locks the mux until mux_control_deselect()
is called. mux_control_deselect() may put the mux in some low power
state or some other user of the mux might select it for other purposes.
These things are probably not happening in the original setting where
this driver was developed, but it is said to be a generic SPI mux.

Also, the mux framework will short out the actual low level muxing
operation when/if that is possible.

Fixes: e9e40543 ("spi: Add generic SPI multiplexer")
Signed-off-by: NPeter Rosin <peda@axentia.se>
Link: https://lore.kernel.org/r/20200525104352.26807-1-peda@axentia.seSigned-off-by: NMark Brown <broonie@kernel.org>

drivers/spi/spi-mux.c

@@ -51,6 +51,10 @@ static int spi_mux_select(struct spi_device *spi)
 	struct spi_mux_priv *priv = spi_controller_get_devdata(spi->controller);
 	int ret;
 
+	ret = mux_control_select(priv->mux, spi->chip_select);
+	if (ret)
+		return ret;
+
 	if (priv->current_cs == spi->chip_select)
 		return 0;
 
@@ -62,10 +66,6 @@ static int spi_mux_select(struct spi_device *spi)
 	priv->spi->mode = spi->mode;
 	priv->spi->bits_per_word = spi->bits_per_word;
 
-	ret = mux_control_select(priv->mux, spi->chip_select);
-	if (ret)
-		return ret;
-
 	priv->current_cs = spi->chip_select;
 
 	return 0;