s390/zcrypt: remove obsolete __constant

__constant_cpu_to_le16 converted to cpu_to_le16

Signed-off-by: Vaishali Thakkar <vthakkar1994@gmail.com>
Signed-off-by: Ingo Tuchscherer <ingo.tuchscherer@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>