sun4i-emac has the ability to print a number of diagnostic messages using
dev_dbg depending on message level settings implemented using netif_msg_*
macros. But there's no way to actually enable them.

Add the ability to switch diagnostic messages on using either a module
parameter debug or ethtool -s <netif> msglvl <flags>.
Signed-off-by: Michael Weiser <michael.weiser@gmx.de>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>