sc16is7xx: move RTS delay to workqueue

Instead of spinning under port->lock let's just sleep
inside the kthread.  It should be equivalent as TX cannot
proceed when thread is blocked.

Signed-off-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>