mfd: madera: Fix potential uninitialised use of variable

regmap_read won't set val to anything if an ACKed bus fails.

Signed-off-by: Stuart Henderson <stuarth@opensource.cirrus.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>