hwrng: stm32 - Fix build with CONFIG_PM

Commit c6a97c42 ("hwrng: stm32 - add support for STM32 HW RNG")
was inadequately tested (actually it was tested quite hard so
incompetent would be a better description that inadequate) and does
not compile on platforms with CONFIG_PM set.

Fix this.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>