net: ethernet: mtk-star-emac: fix error path in RX handling

The dma_addr field in desc_data must not be overwritten until after the
new skb is mapped. Currently we do replace it with uninitialized value
in error path. This change fixes it by moving the assignment before the
label to which we jump after mapping or allocation errors.

Fixes: 8c7bd5a4 ("net: ethernet: mtk-star-emac: new driver")
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com> # build
Signed-off-by: David S. Miller <davem@davemloft.net>