ixgbe: Don't return error in flow control configuration if FC is off

When flow control is disabled, an invalid low/high watermark configuration
should not matter.

Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>