The magnetomer raw values were not being rotated in the same direction as
the scaled values. This meant that if the rotation was set to ROTATION_NONE,
the raw values and scaled values were off by 180 degrees.

mc_pos_control auto: replace min_cruise_speed with cruise_speed_90; take care of case when cruise_speed_90 is exactly in the middle of max and min

The previous method made too optimistic assumptions about the resolution of 32 bit floating point numbers.