mfd: arizona: Correct mask to allow setting micbias external cap

Currently the mask for the external capacitor bit is missing when
writing the MICBIAS config meaning it will never be set this patch fixes
this.

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

drivers/mfd/arizona-core.c

@@ -947,6 +947,7 @@ int arizona_dev_init(struct arizona *arizona)
 		regmap_update_bits(arizona->regmap,
 				   ARIZONA_MIC_BIAS_CTRL_1 + i,
 				   ARIZONA_MICB1_LVL_MASK |
+				   ARIZONA_MICB1_EXT_CAP |
 				   ARIZONA_MICB1_DISCH |
 				   ARIZONA_MICB1_BYPASS |
 				   ARIZONA_MICB1_RATE, val);