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>

drivers/mfd/wm8994-regmap.c

@@ -968,6 +968,7 @@ static bool wm8994_readable_register(struct device *dev, unsigned int reg)
 {
 	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: