USB: fix interaction between different interfaces in an "Option" usb device

Just the serial port in the first interface should control DTR and RTS
lines. This way, the closing of the rest of the ports does not produce a=
hangup in the communication.

Signed-off-by: Miguel Angel Alvarez <ma.alvarez@ziv.es>
Signed-off-by: Matthias Urlichs <matthias@urlichs.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>