USB: option.c: correct DTR behaviour

Setting DTR et al. should work for all interfaces
if you actually pass the interface number. :-P
 
This should help with devices that have important pseudo-serial ports
that aren't on the first interface in the device.

Signed-off-by: Chris Collins <chris@ursys.com.au>
Signed-off-by: Matthias Urlichs <matthias@urlichs.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>