net: mediatek: mtk_cal_txd_req() returns bad value

The code used to also support the PDMA engine, which had 2 packet pointers
per descriptor. Because of this we had to divide the result by 2 and round
it up. This is no longer needed as the code only supports QDMA.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>