iio: max1363: Use devm_regulator_get_optional for optional regulator

In kernel version 3.13, devm_regulator_get() may return no error
if a regulator is undeclared. regulator_get_voltage() will return
-EINVAL if this happens. This causes the driver to fail loading if
the vref regulator is not declared.

Since vref is optional, call devm_regulator_get_optional instead.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: Stable@vger.kernel.org
Signed-off-by: Jonathan Cameron <jic23@kernel.org>