s390/qeth: release cmd buffer in error paths

Whenever we fail before/while starting an IO, make sure to release the
IO buffer. Usually qeth_irq() would do this for us, but if the IO
doesn't even start we obviously won't get an interrupt for it either.

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