tty_port: add "tty_port_open" helper

For the moment this just moves the USB logic over and fixes the 'what if
we open and hangup at the same time' race noticed by Oliver Neukum.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Oliver Neukum <oliver@neukum.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>