bnxt_en: Use dma_rmb() instead of rmb().

Use the weaker but more appropriate dma_rmb() to order the reading of
the completion ring.

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