s390/zcrypt: Move inline function rng_type6cprb_msgx from header to code

s390/zcrypt: Move inline function rng_type6cprb_msgx from header to code

Function rng_type6cprb_msgx() is only used once and thus no need
to provide it in header file any more. Move it at the place within
the code where it is used.

Reviewed-by: Holger Dengler <dengler@linux.ibm.com>
Reviewed-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>