Copter: Keep FIXED mode when WP_YAW_BEHAVIOR is NONE

[tatsuy]

This PR improves the CONDITION_YAW behavior when WP_YAW_BEHAVIOR is set to NONE.

Previously, Yaw Mode would switch to HOLD when reaching a waypoint.
This meant that even if CONDITION_YAW was used, its effect would be reset upon reaching each WAYPOINT. As a result, the vehicle's yaw could potentially drift due to external factors like wind.

With this change, when WP_YAW_BEHAVIOR is NONE and CONDITION_YAW has been used (resulting in FIXED mode), the yaw remains in FIXED mode even after reaching WAYPOINT, maintaining the desired heading as specified by CONDITION_YAW.

Before

After

Settings:

• WP_YAW_BEHAVIOR=0 (None)
• Used CONDITION_YAW with Deg = 10, Speed = 0, Dir = 0, Abs = 0.

Testing:
SITL testing was performed to confirm the behavior. To create a challenging environment for yaw control and to introduce yaw error, the following parameters were used to simulate wind and turbulence.

• SIM_WIND_SPD = 10
• SIM_WIND_TURB = 10

With these settings, the updated implementation successfully maintained the yaw as expected, with FIXED mode persisting throughout WAYPOINT without reverting to HOLD.

[rmackay9]

It's so great to see this fix!