ibmvnic: Fix TX completion error handling

TX completions received with an error return code are not
being processed properly. When an error code is seen, do not
proceed to the next completion before cleaning up the existing
entry's data structures.

Fixes: 032c5e82

 ("Driver for IBM System i/p VNIC protocol")
Signed-off-by: Thomas Falcon <tlfalcon@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>