drm/bridge: cdns-mhdp8546: Fix use of uninitialized variable

'ret' could be uninitialized at the end of the function, although it's
not clear if that can happen in practice.

Fixes: 6a3608ea ("drm: bridge: cdns-mhdp8546: Enable HDCP")
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231103-uninit-fixes-v2-3-c22b2444f5f5@ideasonboard.com