- [ardrone] added daren's mag freeze fix

finally managed to put that piece here. please check. thx
Please fix comments according to the code: 100 counts at 200Hz means re-init after 0.5 seconds, right ?

What about setting the counter at a lower value ? like 30 or 50. It is already unlikely that the mag gives 50 times the same value (unless the internal sampling frequency of the mag is much lower than 200Hz)