net: ethernet: mediatek: Rename NEXT_RX_DESP_IDX to NEXT_DESP_IDX

Rename the NEXT_RX_DESP_IDX macro to NEXT_DESP_IDX, so that it better
can be used for TX ops as well. This will be used in the upcoming
MT7628/88 support (same functionality for RX and TX in this macro).

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: René van Dorst <opensource@vdorst.com>
Cc: Daniel Golle <daniel@makrotopia.org>
Cc: Sean Wang <sean.wang@mediatek.com>
Cc: John Crispin <john@phrozen.org>
Signed-off-by: David S. Miller <davem@davemloft.net>