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>
diff --git a/drivers/mfd/arizona-core.c b/drivers/mfd/arizona-core.c
index 72b3202..bce7c07 100644
--- a/drivers/mfd/arizona-core.c
+++ b/drivers/mfd/arizona-core.c
@@ -947,6 +947,7 @@
 		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);