s390/qeth: re-indent qeth_check_ipa_data()

Pull one level of checking up into qeth_send_control_data_cb(), and
clean up an else-after-return. No functional change.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>