bnxt_en: Fix DMA unmapping of the RX buffers in XDP mode during shutdown.

In bnxt_free_rx_skbs(), which is called to free up all RX buffers during
shutdown, we need to unmap the page if we are running in XDP mode.

Fixes: c61fb99c ("bnxt_en: Add RX page mode support.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>