tg3: Disable multivec mode for 1 MSIX vector

For single vector MSI-X allocations, we do not want to enable
multivector modes.  This patch makes the necessary corrections.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>