soc: amlogic: canvas: Fix meson_canvas_get when probe failed

When probe fails, a platforn_device is still associated to the node,
but dev_get_drvdata() returns NULL.

Handle this case by returning a consistent error.

Fixes: d4983983 ("soc: amlogic: add meson-canvas driver")
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Maxime Jourdan <mjourdan@baylibre.com>
[khilman: fixed minor typo in comment ]
Signed-off-by: Kevin Hilman <khilman@baylibre.com>