greybus: pwm: replace pr_err with dev_err

Replace pr_err with the more descriptive dev_err. Also include the error
code on failure to register the PWM chip.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>

drivers/staging/greybus/pwm.c

@@ -204,7 +204,7 @@ static int gb_pwm_connection_init(struct gb_connection *connection)
 
 	ret = pwmchip_add(pwm);
 	if (ret) {
-		pr_err("Failed to register PWM\n");
+		dev_err(&connection->dev, "failed to register PWM: %d\n", ret);
 		goto out_err;
 	}