tg3: Adjust RSS ring allocation strategies

When multivector RSS is enabled, the first interrupt vector is only used
to report link interrupts and error conditions.  This patch changes the
code so that rx and tx ring resources are not allocated for this vector.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>