bnxt_en: Don't use DEFINE_DMA_UNMAP_ADDR to store DMA address in RX path.

To support XDP_TX, we need the RX buffer's DMA address to transmit the
packet.  Convert the DMA address field to a permanent field in
bnxt_sw_rx_bd.

Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>