mfd: wm5102: Mark register write sequencer control 3 readable

During init the core checks if the wm5102 has finished starting by reading
register 0x19 and looking at the value. This read always fails since this
is not a readable register, mark it as being one. While we're at it provide
a constant for the register name (as supplied by Charles Keepax).

Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
diff --git a/drivers/mfd/arizona-core.c b/drivers/mfd/arizona-core.c
index 10a0cb9..7eabc36 100644
--- a/drivers/mfd/arizona-core.c
+++ b/drivers/mfd/arizona-core.c
@@ -784,7 +784,8 @@
 	/* Ensure device startup is complete */
 	switch (arizona->type) {
 	case WM5102:
-		ret = regmap_read(arizona->regmap, 0x19, &val);
+		ret = regmap_read(arizona->regmap,
+				  ARIZONA_WRITE_SEQUENCER_CTRL_3, &val);
 		if (ret != 0)
 			dev_err(dev,
 				"Failed to check write sequencer state: %d\n",