net: stmmac: create one debugfs dir per net-device

stmmac DebugFS entries are currently global to the driver. As a result,
having more than one stmmac device in the system creates the following
error:
* ERROR stmmaceth, debugfs create directory failed
* stmmac_hw_setup: failed debugFS registration

This also results in being able to access the debugfs information for
the first registered device only.

This patch changes the debugfs structure to have one sub-directory per
net-device. Files under "/sys/kernel/debug/stmmaceth" will now show-up
under /sys/kernel/debug/stmmaceth/ethN/.

Signed-off-by: Mathieu Olivari <mathieu@codeaurora.org>
Signed-off-by: David S. Miller <davem@davemloft.net>