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>

drivers/char/hw_random/picoxcell-rng.c

@@ -33,7 +33,7 @@
 
 static void __iomem *rng_base;
 static struct clk *rng_clk;
-struct device *rng_dev;
+static struct device *rng_dev;
 
 static inline u32 picoxcell_trng_read_csr(void)
 {