drivers: sensor: lsm303dlhc_mag: Fix gain factor

The magnetometer on the LSM303DLHC has a different gain conversion
factor for LSB to Gauss for the Z axis than it does for X, Y. This
commit takes into account the different conversion factors, and
adds the correct coefficients for each gain setting and axis.

Signed-off-by: Kevin Townsend <kevin.townsend@linaro.org>