bnxt_en: Disable RDMA support on the 57500 chips.

There is no RDMA support on 57500 chips yet, so prevent bnxt_re from
registering on these chips.  There is intermittent failure if bnxt_re
is allowed to register and proceed with RDMA operations.

Fixes: 1ab968d2 ("bnxt_en: Add PCI ID for BCM57508 device.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>