net: stmmac: Move debugfs init/exit to ->probe()/->remove()

Setting up and tearing down debugfs is current unbalanced, as seen by
this error during resume from suspend:

    [  752.134067] dwc-eth-dwmac 2490000.ethernet eth0: ERROR failed to create debugfs directory
    [  752.134347] dwc-eth-dwmac 2490000.ethernet eth0: stmmac_hw_setup: failed debugFS registration

The imbalance happens because the driver creates the debugfs hierarchy
when the device is opened and tears it down when the device is closed.
There's little gain in that, and it could be argued that it is even
surprising because it's not usually done for other devices. Fix the
imbalance by moving the debugfs creation and teardown to the driver's
->probe() and ->remove() implementations instead.

Note that the ring descriptors cannot be read while the interface is
down, so make sure to return an empty file when the descriptors_status
debugfs file is read.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Jose Abreu <joabreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>