bnxt_en: Fix ethtool self-test loopback.

The current code has 2 problems.  It assumes that the RX ring for
the loopback packet is combined with the TX ring.  This is not
true if the ethtool channels are set to non-combined mode.  The
second problem is that it won't work on 57500 chips without
adjusting the logic to get the proper completion ring (cpr) pointer.
Fix both issues by locating the proper cpr pointer through the RX
ring.

Fixes: e44758b7 ("bnxt_en: Use bnxt_cp_ring_info struct pointer as parameter for RX path.")
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>