button: qcom-pmic: fix some error checking

The pmic_reg_read() function can return errors.  Add a check for that.

Fixes: 4e8aa006 ("button: qcom-pmic: introduce Qualcomm PMIC button driver")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>