cdc-wdm: cleanup debug messages

Dynamic debugging will already add the function (and the line number)
to a debug message if one requests that. It makes no sense to add
them unconditionally in a driver.

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>