iio:trigger: Use to_iio_trigger() instead of dev_get_drvdata()

Use to_iio_trigger(dev) instead of dev_get_drvdata(dev). Both will return the
trigger which belongs to the device, but the the first on is a bit more
lightweight.

Since this is the last location where we used dev_get_drvdata() for retrieving
the trigger there is no need anymore to assign the the trigger to the devices
drvdata, so we can remove that as well.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>