iio: Make return value check for set_trigger_state() consistent

In `iio_trigger_detach_poll_func()` the return value from
`trig->ops->set_trigger_state(trig, false)` is checked with `if (ret)`.
However, in `iio_trigger_attach_poll_func()` it is checked with
`if (ret < 0)`. Resolve this mismatch by only checking for `if (ret)` in
both places.

Signed-off-by: Waqar Hameed <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

drivers/iio/industrialio-trigger.c

···
 	/* Enable trigger in driver */
 	if (trig->ops && trig->ops->set_trigger_state && notinuse) {
 		ret = trig->ops->set_trigger_state(trig, true);
-		if (ret < 0)
+		if (ret)
 			goto out_free_irq;
 	}