s390/hypfs: stop using ENOSYS error code

ENOSYS should only be returned to userspace when a syscall
is not implemented. The only known user, 'hyptop' is not explicitely
checking for -ENOSYS, therefore use EOPNOTSUPP instead. It is very
unlikely that there are other users, so this change should have no
impact on userspace.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>