hmc5883l: fixed type cast in update()

Signed-off-by: Mihai Tudor Panu <mihai.t.panu@intel.com>
hmc5883l: changed to standard heading and added declination

Signed-off-by: Mihai Tudor Panu <mihai.t.panu@intel.com>
hmc5883l: use int16_t as coordinate type throughout

Signed-off-by: Brendan Le Foll <brendan.le.foll@intel.com>