s390/qeth: clean up stale buffer state documentation

We don't keep track of Input Buffer states, so remove the comments that
make it sound like the qeth_qdio_buffer_states enum applies to
Input Buffers.

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