MIPS: Octeon: Fix a typo in #define OCTOEN_SERIAL_LEN

It should be OCTEON_SERIAL_LEN.
Update the #define and use it accordingly

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: ralf@linux-mips.org
Cc: jhogan@kernel.org
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: kernel-janitors@vger.kernel.org