iio: mxs-lradc: Fix misuse of iio->trig

The struct iio_dev .trig field is to be used only by the IIO core,
the driver shall not fill this field. This fixes ugly crash when
the driver is compiled as a module and the module is rmmod'd.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Jonathan Cameron <jic23@kernel.org>
Cc: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>