mfd: mc13xxx: Fix a missing check of a register-read failure

When mc13xxx_reg_read() fails, "old_adc0" is uninitialized and will
contain random value. Further execution uses "old_adc0" even when
mc13xxx_reg_read() fails.
The fix checks the return value of mc13xxx_reg_read(), and exits
the execution when it fails.

Signed-off-by: Kangjie Lu <kjlu@umn.edu>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/mc13xxx-core.c

@@ -274,7 +274,9 @@ int mc13xxx_adc_do_conversion(struct mc13xxx *mc13xxx, unsigned int mode,
 
 	mc13xxx->adcflags |= MC13XXX_ADC_WORKING;
 
-	mc13xxx_reg_read(mc13xxx, MC13XXX_ADC0, &old_adc0);
+	ret = mc13xxx_reg_read(mc13xxx, MC13XXX_ADC0, &old_adc0);
+	if (ret)
+		goto out;
 
 	adc0 = MC13XXX_ADC0_ADINC1 | MC13XXX_ADC0_ADINC2 |
 	       MC13XXX_ADC0_CHRGRAWDIV;