hw_random: free rng_buffer at module exit

rng-core module allocates rng_buffer by kmalloc() since commit
f7f154f1. But this buffer won't be
freed and there is a memory leak possibility at module exit.

Signed-off-by: Satoru Takeuchi <satoru.takeuchi@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>