hwrng: stm32 - fix unbalanced pm_runtime_enable

commit af0d4442 upstream.

No remove function implemented yet in the driver.
Without remove function, the pm_runtime implementation
complains when removing and probing again the driver.

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>