s390/kaslr: Fix casts in get_random

s390/kaslr: Fix casts in get_random

Clang warns:

../arch/s390/boot/kaslr.c:78:25: warning: passing 'char *' to parameter
of type 'const u8 *' (aka 'const unsigned char *') converts between
pointers to integer
types with different sign [-Wpointer-sign]
                                  (char *) entropy, (char *) entropy,
                                                    ^~~~~~~~~~~~~~~~
../arch/s390/include/asm/cpacf.h:280:28: note: passing argument to
parameter 'src' here
                            u8 *dest, const u8 *src, long src_len)
                                                ^
2 warnings generated.

Fix the cast to match what else is done in this function.

Fixes: b2d24b97b2a9 ("s390/kernel: add support for kernel address space layout randomization (KASLR)")
Link: https://github.com/ClangBuiltLinux/linux/issues/862
Link: https://lkml.kernel.org/r/20200208141052.48476-1-natechancellor@gmail.com
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>