qeth: return zero from reply callback functions

Reply callback functions in qeth should return zero if command
response consists of one part only, otherwise qeth continues
waiting for further parts of the command response.

Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: Frank Blaschka <frank.blaschka@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>