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>