hwrng: st - Fix W=1 unused variable warning

[ Upstream commit ad237562 ]

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: 501e197a ("hwrng: st - keep clock enabled while hwrng is registered")
Signed-off-by: Sasha Levin <sashal@kernel.org>