xlnx_dp: drop unsupported AUXCommand in xlnx_dp_aux_set_command

In xlnx_dp_aux_set_command, when the command leads to the default
branch, xlxn-dp will abort and then crash.

This patch removes this abort and drops this operation.

Fixes: 58ac482a ("introduce xlnx-dp")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/411


Reported-by: Qiang Liu <cyruscyliu@gmail.com>
Tested-by: Qiang Liu <cyruscyliu@gmail.com>
Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Qiang Liu <cyruscyliu@gmail.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Frederic Konrad <fkonrad@amd.com>
Message-Id: <20220808080116.2184881-1-cyruscyliu@gmail.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>