staging:iio:accel:lis3l02dq add symmetry to check for presence of trigger.

Checking indio_dev->modes is uggly and not symmetric with the conditions
on whether triggers are allowed in the first place.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>