hwrng: mtk - handle devm_pm_runtime_enable errors

[ Upstream commit 522a242a18adc5c63a24836715dbeec4dc3faee1 ]

Although unlikely, devm_pm_runtime_enable() call might fail, so handle
the return value.

Fixes: 78cb66caa6ab ("hwrng: mtk - Use devm_pm_runtime_enable")
Signed-off-by: Ovidiu Panait <ovidiu.panait.oss@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Sasha Levin <sashal@kernel.org>