ixgbevf: Fix panic when loading driver

This patch addresses a kernel panic seen when setting up the interface.
Specifically we see a NULL pointer dereference on the Tx descriptor cleanup
path when enabling interrupts.  This change corrects that so it cannot
occur.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Acked-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>