drm/crc-debugfs: Fix memleak in crc_control_write

drm/crc-debugfs: Fix memleak in crc_control_write

When verify_crc_source() fails, source needs to be freed.
However, current code is returning directly and ends up
leaking memory.

Fixes: c861f6aad979 ("drm: crc: Introduce verify_crc_source callback")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
[danvet: change Fixes: tag per Laurent's review]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200819082228.26847-1-dinghao.liu@zju.edu.cn