MIPS: Annotate set_except_vector with __init

All call sites of set_except_vector are already annotated with __init, so
annotate that one too.

Signed-off-by: Regards, Florian Fainelli <florian@openwrt.org>
To: linux-mips@linux-mips.org
To: David Daney <ddaney@caviumnetworks.com>
Patchwork: http://patchwork.linux-mips.org/patch/888/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>