The EKF2_MAG_DECL parameter is not being saved in disarm

[priseborough]

Describe the bug

The option to save the initial declination loaded from the geo library to the EKF2_MAG_DECL parameter on disarm so that it can be loaded as the startup default for the next power cycle is supposed to be controlled via setting of the EKF2_DECL_TYPE parameter. However inspection of the code shows that this capability is now missing from ekf2_main.cpp

It is supposed to save the value obtained from the copy_mag_decl_deg(float *val) accessor function to the EKF2_MAG_DECL parameter if bit position 1 in EKF2_DECL_TYPE is set to true.

Additionally the comment and function name on that accessor function is misleading. It should read

// Get the value of magnetic declination in degrees to be saved for use at the next startup
// At the next startup, set param.mag_declination_deg to the value saved
void get_mag_decl_deg(float *val)

A separate PR will be raised against the ecl library to fix that.