ibmvnic: Fix TX descriptor tracking

With the recent change, transmissions that only needed
one descriptor were being missed. The result is that such
packets were tracked as outstanding transmissions but never
removed when its completion notification was received.

Fixes: ffc385b9 ("ibmvnic: Keep track of supplementary TX descriptors")
Signed-off-by: Thomas Falcon <tlfalcon@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>