tty: serial: fix typo "SERIAL_S3C2412"

The Kconfig symbol SERIAL_S3C2412 got removed in commit
da121506

 ("serial: samsung: merge
probe() function from all SoC specific extensions"). But it also added a
last reference to that symbol. The commit and the tree make clear that
CPU_S3C2412 should have been used instead.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>