mfd: intel_soc_pmic_core: Fix unchecked return value

unction devm_regmap_init_i2c() returns an ERR_PTR on errors, and its
return value should be checked before it is dereferenced. However, in
function intel_soc_pmic_i2c_probe(), the return value of function
devm_regmap_init_i2c() is used without validation. This patch fixes it.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>