net: stmmac: dwmac-rk: Fix ethernet on rk3399 based devices

Commit 2d26f6e3 ("net: stmmac: dwmac-rk: fix unbalanced pm_runtime_enable warnings")
while getting rid of a runtime PM warning ended up breaking ethernet
on rk3399 based devices. By dropping an extra reference to the device,
the commit ends up enabling suspend / resume of the ethernet device -
which appears to be broken.

While the issue with runtime pm is being investigated, partially
revert commit 2d26f6e3 to restore the network on rk3399.

Fixes: 2d26f6e3

 ("net: stmmac: dwmac-rk: fix unbalanced pm_runtime_enable warnings")
Suggested-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Punit Agrawal <punitagrawal@gmail.com>
Cc: Michael Riesch <michael.riesch@wolfvision.net>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20210929135049.3426058-1-punitagrawal@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>