tg3: Fix tx mailbox initialization

If a device supports MSI-X interrupts, the driver assumes TSS will be
available.  This is not true for the 57765.  This patch changes the code
so that only the default tx mailbox is initialized if TSS is not
available.

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