i40e: allow FD SB if MFP mode only has 1 partition

Even though the device might be in MFP mode, if there's only one partition
enabled, then we still have plenty of interrupts for managing the Flow
Directory Sideband activity.  This patch enables FD SB in this case.
This patch also reverses the sense of the conditional in order to remove
the negative logic.

Change-ID: I9edf211a6219fc8d159b4be9964f9fd7f4e00bc0
Signed-off-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>