usbnet: allow type check of devdbg arguments in non-debug build

Improve usbnet's devdbg to always type-check diagnostic arguments,
like dev_dbg (device.h).  This makes no change to the resulting size of
usbnet modules.

This patch also removes an #ifdef DEBUG directive from rndis_wlan so
it's devdbg statements are always type-checked at compile time.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: David S. Miller <davem@davemloft.net>