bnxt_en: Add bp->rx_dir field for rx buffer DMA direction.

When driver is running in XDP mode, rx buffers are DMA mapped as
DMA_BIDIRECTIONAL.  Add a field so the code will map/unmap rx buffers
according to this field.

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