net: Delete trailing semi-colon from definition of netdev_WARN()

Macro definitions should not normally end with a semi-colon, as this
makes it dangerous to use them an if...else statement.  Happily this
has not happened yet.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>