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 <waqar.hameed@axis.com>
Link: https://lore.kernel.org/r/pndv8eojdey.fsf@axis.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>