net: ethernet: mtk_eth_soc: fix uninitialized variable

Variable dma_addr in function mtk_poll_rx can be uninitialized on
some of the error paths. In practise this doesn't matter, even random
data present in uninitialized stack memory can safely be used in the
way it happens in the error path.

However, in order to make Smatch happy make sure the variable is
always initialized.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: David S. Miller <davem@davemloft.net>