bnxt_en: Fix _bnxt_get_max_rings() for 57500 chips.

The CP rings are accounted differently on the new 57500 chips.  There
must be enough CP rings for the sum of RX and TX rings on the new
chips.  The current logic may be over-estimating the RX and TX rings.

The output parameter max_cp should be the maximum NQs capped by
MSIX vectors available for networking in the context of 57500 chips.
The existing code which uses CMPL rings capped by the MSIX vectors
works most of the time but is not always correct.

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