drm: xlnx: zynqmp: fix a memset in zynqmp_dp_train()

The dp->train_set[] for this driver is only two characters, not four so
this memsets too much.  Fortunately, this ends up corrupting a struct
hole and not anything important.

Fixes: d76271d2

 ("drm: xlnx: DRM/KMS driver for Xilinx ZynqMP DisplayPort Subsystem")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/YGLwCBMotnrKZu6P@mwanda