USB: serial: fix console error reporting

Do not report errors in write path if port is used as a console as this
may trigger the same error (and error report) resulting in a loop.

Reported-by: Stephen Hemminger <shemminger@vyatta.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>