RC_Channel: To nullptr from NULL.
AC_Fence: To nullptr from NULL.
AC_Avoidance: To nullptr from NULL.
AC_PrecLand: To nullptr from NULL.
DataFlash: To nullptr from NULL.
SITL: To nullptr from NULL.
GCS_MAVLink: To nullptr from NULL.
DataFlash: To nullptr from NULL.
AP_Compass: To nullptr from NULL.
Global: To nullptr from NULL.
Global: To nullptr from NULL.
We are not doing any dma here, it's just an SPI transaction. Name them
only rx/tx (although io_packet_tx/io_packet_rx could be another option).
This also zero-initialize the struct to keep valgrind happy about not
calling ioctl() with uninitialized variables.