AP_NavEKF: Fix bug that could cause divergence in magnetic field states

Leaving the off-diagonal terms in the state covariance matrix frozen
when magnetic fields state corrections were inhibited was incorrect as
these terms capture the correlation between the errors in the various
states, and need to either be continuously updated, or zeroed.

An theoretically more correct approach is to to turn the process on these
states to zero when corrections are being inhibited, however this can be
numerically less stable.