net: ocelot: use dma_unmap_addr to get tx buffer dma_addr

dma_addr was declared using DEFINE_DMA_UNMAP_ADDR() which requires to
use dma_unmap_addr() to access it.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 753a026c ("net: ocelot: add FDMA support")
Signed-off-by: Clément Léger <clement.leger@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>