tty: rocket: Remove private close_wait

This driver's private completion variable, close_wait, is no longer
used for wait since "tty: Remove ASYNC_CLOSING checks in open()/hangup";
remove.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>