Replace pm_runtime_enable with the devres-enabled version which
can trigger pm_runtime_disable.

Otherwise, the below appears during reload driver.

mtk_rng 1020f000.rng: Unbalanced pm_runtime_enable!

Fixes: 81d2b345 ("hwrng: mtk - add runtime PM support")
Cc: <stable@vger.kernel.org>
Suggested-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Guoqing Jiang <guoqing.jiang@canonical.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>