s390/qdio: use get_buf_state() in debug_get_buf_state()

For a 1-SBAL state inspection, use the corresponding helper.
No functional change, just reducing the number of immediate callers to
get_buf_states().

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>