qeth: Fix IPA_CMD_QIPASSIST return code handling

Return codes of IPA_CMD_QIPASSIST are not checked, especially the ones which
indicate that the command is not supported. As a result, the device driver
would not enable all available features on older card generations.
This patch adds proper checking and sets the bare minimum in the supported
functions flags to avoid follow-on errors.

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