hwrng: optee - remove redundant initialization to variable rng_size

Variable rng_size is being initialized with a value that is never read,
the variable is being re-assigned later on. The initialization is
redundant and can be removed.

Cleans up cppcheck warning:
Variable 'rng_size' is assigned a value that is never used.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>