USB: serial: cp210x: disable break signalling on CP2105 SCI

Only the first UART interface (ECI) on CP2105 supports break signalling.

Return an error on requests for break state changes for the second
interface (SCI) to avoid transmitting a garbage character and waiting
when break is not supported.

Tested-by: Corey Minyard <cminyard@mvista.com>
Signed-off-by: Johan Hovold <johan@kernel.org>