iio: lsiio: fix error code handling error

commit acf50b3586f8d8a7530b905e111dda41876d38f4
"tools:iio:lsiio: add error handling"
introduced error handling of errors returned from
read_sysfs_string(), but with a simple if (retval),
missing the fact that these functions return a positive
value if the read was successful.

As a result lsiio regresses and does not show any
devices on my filesystem. Fix this by checking for
only negative error codes.

Cc: Hartmut Knaack <[e-mail]>
Signed-off-by: Linus Walleij <[e-mail]>
Acked-by: Hartmut Knaack <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

tools/iio/lsiio.c

···
 		return -EINVAL;
 
 	ret = read_sysfs_string("name", dev_dir_name, name);
-	if (ret)
+	if (ret < 0)
 		return ret;
 
 	printf("Device %03d: %s\n", dev_idx, name);
···
 		return -EINVAL;
 
 	ret = read_sysfs_string("name", dev_dir_name, name);
-	if (ret)
+	if (ret < 0)
 		return ret;
 
 	printf("Trigger %03d: %s\n", dev_idx, name);