hwrng: st - Improve FIFO size/depth description

The original representation of FIFO size in the driver coupled with the
ambiguity in the documentation meant that it was easy to confuse readers.
This lead to a false positive BUG-find and subsequently time wastage
debugging this phantom issue.

Hopefully this patch can prevent future readers from falling into the
same trap.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>