mfd: wm8994: Mark MICBIAS register as readable

Omitted in the regmap conversion.

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..47743fa 100644
--- a/drivers/mfd/wm8994-regmap.c
+++ b/drivers/mfd/wm8994-regmap.c
@@ -968,6 +968,7 @@
 {
 	switch (reg) {
 	case WM8994_DC_SERVO_READBACK:
+	case WM8994_MICBIAS:
 	case WM8994_WRITE_SEQUENCER_CTRL_1:
 	case WM8994_WRITE_SEQUENCER_CTRL_2:
 	case WM8994_AIF1_ADC2_LEFT_VOLUME: