bnxt_en: Cap the msix vector with the max completion rings.

The current code enables up to the maximum MSIX vectors in the PCIE
config space without considering the max completion rings available.
An MSIX vector is only useful when it has an associated completion
ring, so it is better to cap it.

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