pinctrl: madera: Fix uninitialized variable bug in madera_mux_set_mux

There is a potential execution path in which variable *ret* is checked
in an IF statement, and then its value is used to report an error at
line 659 without being properly initialized previously:

659 if (ret)
660	dev_err(priv->dev, "Failed to write to 0x%x (%d)\n", reg, ret);

Fix this by initializing variable *ret* to 0 in order to
avoid unpredictable or unintended results.

Addresses-Coverity-ID: 1471969 ("Uninitialized scalar variable")
Fixes: 218d72a7 ("pinctrl: madera: Add driver for Cirrus Logic Madera codecs")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>