For HMC5883 :
#define HMC5883_CONVERSION_INTERVAL (1000000 / 110) /* Notice !!! microseconds, must not lower than 100 (sensor.cpp L:943) */

Signed-off-by: cctsao1008 <chiacheng.tsao@gmail.com>
Sensor stuffs, QgroundControl test passed, update submodule "Nuttx".

Signed-off-by: cctsao1008 <chiacheng.tsao@gmail.com>
Merge branch 'sensors' into alpha