bnxt_en: Add helper functions to get firmware CP ring ID.

On the new 57500 chips, getting the associated CP ring ID associated with
an RX ring or TX ring is different than before.  On the legacy chips,
we find the associated ring group and look up the CP ring ID.  On the
57500 chips, each RX ring and TX ring has a dedicated CP ring even if
they share the MSIX.  Use these helper functions at appropriate places
to get the CP ring ID.

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