mfd: Correct readability of WM8994 DC servo 4E register

It should be marked as readable but wasn't, breaking DC servo operation.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
diff --git a/drivers/mfd/wm8994-regmap.c b/drivers/mfd/wm8994-regmap.c
index c598ae6..bc0c509 100644
--- a/drivers/mfd/wm8994-regmap.c
+++ b/drivers/mfd/wm8994-regmap.c
@@ -806,6 +806,7 @@
 	case WM8994_DC_SERVO_2:
 	case WM8994_DC_SERVO_READBACK:
 	case WM8994_DC_SERVO_4:
+	case WM8994_DC_SERVO_4E:
 	case WM8994_ANALOGUE_HP_1:
 	case WM8958_MIC_DETECT_1:
 	case WM8958_MIC_DETECT_2: