iio: magnetometer: mmc35240: Fix function header formatting

All lines in the header must start with " *".

Fixes the following W=1 kernel build warning(s):

 drivers/iio/magnetometer/mmc35240.c:304: warning: bad line:                             compensation for output value.

Cc: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>