iio: magnetometer: mmc35240: NULL terminate attribute array

This avoid nasty crashes when registering the IIO device.

Fixes: abeb6b1e ("iio: magnetometer: Add support for MEMSIC MMC35240")
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>