LandDetector: fix vehicle flight time

The problem was signed vs unsigned mixing together with 64bit conversion:
int32_t flight_time = ...;
uint64_t _total_flight_time = ...;
_total_flight_time |= flight_time;
When flight_time is negative, the last line would first extend the sign
bit to the upper 32bits of the 64bit uint and then apply the bitwise OR.
The fix is to use an unsigned 32 bit value.