iio: magnetometer: mmc35240: NULL terminate attribute array

This avoid nasty crashes when registering the IIO device.

Fixes: abeb6b1e7b ("iio: magnetometer: Add support for MEMSIC MMC35240")
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/magnetometer/mmc35240.c

@@ -125,6 +125,7 @@ static const struct iio_chan_spec mmc35240_channels[] = {
 
 static struct attribute *mmc35240_attributes[] = {
 	&iio_const_attr_sampling_frequency_available.dev_attr.attr,
+	NULL
 };
 
 static const struct attribute_group mmc35240_attribute_group = {