net: ethernet: mtk-star-emac: remove unused variable

The desc pointer is set but not used. Remove it.

Reported-by: kbuild test robot <lkp@intel.com>
Fixes: 8c7bd5a4 ("net: ethernet: mtk-star-emac: new driver")
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>