hwrng: pixocel - Staticize 'rng_dev'

'rng_dev' is used only in this file.
Fix the following sparse warning:

drivers/char/hw_random/picoxcell-rng.c:36:15: warning: symbol 'rng_dev' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>