TTY: isdn, use tty_port_block_til_ready helper

This removes a bunch of duplicated code which does the same as
tty_port_block_til_ready does.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>