hwrng: st - Fix W=1 unused variable warning

[ Upstream commit ad23756271d5744a0a0ba556f8aaa70e358d5aa6 ]

This patch fixes an unused variable warning when this driver is
built-in with CONFIG_OF=n.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Stable-dep-of: 501e197a02d4 ("hwrng: st - keep clock enabled while hwrng is registered")
Signed-off-by: Sasha Levin <sashal@kernel.org>