examples/power, examples/uart_loopback: fix build warnings

power_main.c:210:49: warning: passing argument 4 of 'task_create' from incompatible pointer type
	Fix: function 'start_pm_test(void)' must be of 'main_t' type.

uart_loopback.c:97:12: warning: assignment to 'char *' from incompatible pointer type
	Fix: assignment to pointer 'char *read_ptr' should be typecast to 'char *'

	- return statement in function 'uart_loopback_task' was missing.
	- removed unused variable 'ret' in function 'uart_loopback_main'

apps/examples/power/power_main.c

@@ -112,7 +112,7 @@ static int pm_suspend_resume_test(void)
 	return 0;
 }
 
-static int start_pm_test(void)
+static int start_pm_test(int argc, char *argv[])
 {
 	pthread_t suspend_resume_tid;
 	int ret;

apps/examples/uart_loopback/uart_loopback.c

@@ -94,7 +94,7 @@ static int uart_rx_loop(void)
 
 	while (is_running) {
 		rx_test_count++;
-		read_ptr = &read_buf;
+		read_ptr = (char *)&read_buf;
 		remain_size = TEST_STR_LEN;
 
 		while (0 < remain_size) {
@@ -173,6 +173,7 @@ static int uart_loopback_task(int argc, char *argv[])
 	}
 
 	printf("######################### UART loopback test END ###########################\n");
+	return ret;
 }
 
 
@@ -182,7 +183,6 @@ int main(int argc, FAR char *argv[])
 int uart_loopback_main(int argc, char **argv)
 #endif
 {
-	int ret;
 	int pid;
 
 	if (argc != 2 || strncmp(argv[1], "help", 5) == 0) {