bnxt_en: Add bnxt_get_avail_cp_rings_for_en() helper function.

The available CP rings are calculated differently on the new 57500
chips, so add this helper to do this calculation correctly.  The
VFs will be assigned these available CP rings.

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