qeth: fix handling of IPA return codes

Function that executes IPA commands returns the result code from the
IPA response block. If non-negative, it needs to be transformed into
errno-compatible code before returning to the caller.

Signed-off-by: Eugene Crosser <Eugene.Crosser@ru.ibm.com>
Signed-off-by: Ursula Braun <ursula.braun@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>