ionic: better dma-map error handling

Fix up a couple of small dma_addr handling issues
  - don't double-count dma-map-err stat in ionic_tx_map_skb()
    or ionic_xdp_post_frame()
  - return 0 on error from both ionic_tx_map_single() and
    ionic_tx_map_frag() and check for !dma_addr in ionic_tx_map_skb()
    and ionic_xdp_post_frame()
  - be sure to unmap buf_info[0] in ionic_tx_map_skb() error path
  - don't assign rx buf->dma_addr until error checked in ionic_rx_page_alloc()
  - remove unnecessary dma_addr_t casts

Reviewed-by: Brett Creeley <brett.creeley@amd.com>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>