This resolves issue #1021 in which LAND mode could descend at the
PILOT_VELZ rate instead of the WPNAV_SPEED_DN
Pilot defined acceleration is used for AltHold, AutoTune , Circle,
Hybrid, Loiter, OF_Loiter and Sport flight modes
Waypoint Nav (ie. AutoPilot) acceleration is used for Auto, Land, RTL
This ensures invalid pilot desired accelerations are cleared from the
loiter controller. This is probably not strictly necessary because the
vehicle should switch out of Loiter and into RTL during failsafe.
