regulator: pwm-regulator: Fix ' comparison between signed and unsigned integer' warning

drivers/regulator/pwm-regulator.c:
  In function ‘pwm_regulator_init_table’:
drivers/regulator/pwm-regulator.c:171:14:
  warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
Signed-off-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NMark Brown <broonie@kernel.org>

drivers/regulator/pwm-regulator.c

@@ -163,7 +163,7 @@ static int pwm_regulator_init_table(struct platform_device *pdev,
 {
 	struct device_node *np = pdev->dev.of_node;
 	struct pwm_voltages *duty_cycle_table;
-	int length = 0;
+	unsigned int length = 0;
 	int ret;
 
 	of_find_property(np, "voltage-table", &length);