s390/crypto: provide correct file mode at device register.

When the prng device driver calls misc_register() there is the possibility
to also provide the recommented file permissions. This fix now gives
useful values (0644) where previously just the default was used (resulting
in 0600 for the device file).

Signed-off-by: Harald Freudenberger <freude@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>