tg3: Fix single MSI-X vector coalescing

The interrupt coalescing setup code used the TG3_FLG2_USING_MSIX flag to
determine whether or not to configure the rx coalescing parameters.
This is incorrect for the single MSI-X vector case.  This patch changes
the code to look at the TG3_FLG3_ENABLE_RSS instead.

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