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).