usb: cdc-wdm: remove logically dead code

Remove logically dead code.
'cntr' is always equal to zero when the following line of code is executed:
rv = cntr ? cntr : -EAGAIN;

Addresses-Coverity-ID: 113227
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Acked-by: Oliver Neukum <oneukum@suse.com>
Reviewed-by: Peter Senna Tschudin <peter.senna@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>