iio: magnetometer: mmc35240: Fix sensitivity on z-axis

Datasheet says (Page 2) that typical value for sensitivity
for 16 bits mode on Z-axis is 770. Anyhow, looking at the
input driver provided by Memsic the value for MMC35240 is
1024.

Also, testing shows that using 1024 for Z-axis senzitivity
offers better results.

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>