tty: serial: fix typo "ARCH_S5P6450"

commit 827aa0d36d486f359808c8fb931cf7a71011a09d upstream.

This could have been either ARCH_S5P64X0 or CPU_S5P6450. Looking at
commit 2555e663b367b8d555e76023f4de3f6338c28d6c ("ARM: S5P64X0: Add UART
serial support for S5P6450") - which added this typo - makes clear this
should be CPU_S5P6450.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>