s390/dasd: replace seq_printf by seq_puts

Fix checkpatch warnings:
"WARNING: Prefer seq_puts to seq_printf"

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Stefan Haberland <stefan.haberland@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>