[S390] prng: fix pointer arithmetic

The git commit c708c57e fixed the
access beyond the end of the stack in prng_seed but the pointer
arithmetic is still incorrect. The calculation has been off by
a factor of 64, now it is only off by a factor of 8. prng_seed
is called with a maximum of 16 for nbytes, small enough that
the incorrect calculation stays insides the limits of the stack.
Place parentheses for correct pointer arithmetic.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>