media: synopsys: hdmirx: Renamed frame_idx to sequence

This variable is used to fill the v4l2_buffer.sequence, let's name it
the exact same way to reduce confusion.

No functional changes.

Fixes: 7b59b132 ("media: platform: synopsys: Add support for HDMI input driver")
Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>