Revert "serial: of-serial: fetch line number from DT"

This reverts commit 1bd83245.

To quote Olof:
	This commit broke a whole lot of tegra boards in last night's
	-next here. In particular, I've been looking at
	tegra20-seaboard, which now doesn't boot with console any more.

Breaking existing systems is bad.

Reported-by: Olof Johansson <olof@lixom.net>
Cc: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>