s390/qdio: remove always-true condition

buf_in_between() gets passed q->u.in.ack_start as 'bufnr' parameter.

The ack_start always ranges between 0 and QDIO_MAX_BUFFERS_PER_Q - 1,
so the subsequent check will always return true. Remove it.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>