defxx: Use netdev_alloc_skb consistently

Switch the two remaining places across the driver that use dev_alloc_skb
to netdev_alloc_skb.  Another place has already been converted to use
__netdev_alloc_skb, no idea why these two have been left behind.

Reported-by: Robert Coerver <Robert.Coerver@ll.mit.edu>
Tested-by: Robert Coerver <Robert.Coerver@ll.mit.edu>
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>