i40e: remove redundant check on vsi->active_vlans

active_vlans is an unsigned long array, hence a null check on this
array is superfluous and can be removed.

Detected with static analysis by smatch:

drivers/net/ethernet/intel/i40e/i40e_debugfs.c:386
  i40e_dbg_dump_vsi_seid() warn: this array is probably
  non-NULL. 'vsi->active_vlans'

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-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>