stmmac: no need to check return value of debugfs_create functions

commit 8d72ab119f42f25abb393093472ae0ca275088b6 upstream.

When calling debugfs functions, there is no need to ever check the
return value.  The function can work or not, but the code logic should
never do something different based on this.

Because we don't care about the individual files, we can remove the
stored dentry for the files, as they are not needed to be kept track of
at all.

Cc: Giuseppe Cavallaro <peppe.cavallaro@st.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Jose Abreu <joabreu@synopsys.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: netdev@vger.kernel.org
Cc: linux-stm32@st-md-mailman.stormreply.com
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Hugo SIMELIERE <hsimeliere.opensource@witekio.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Stable-dep-of: 474a31e13a4e ("net: stmmac: fix notifier registration")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>