s390/hmcdrv: constify hmcdrv_ftp_ops structs

Constifies hmcdrv_ftp_ops structures in s390's char
driver since they are not modified after their
initialization.

Detected and found using Coccinelle.

Suggested-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Aya Mahfouz <mahfouz.saif.elyazal@gmail.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>