s390/qeth: handle error when updating TX queue count

netif_set_real_num_tx_queues() can return an error, deal with it.

Fixes: 73dc2daf ("s390/qeth: add TX multiqueue support for OSA devices")
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>