Fix NULL pointer access in BMIPS3300 RAC flush.

Fixes: 738a3f79 ("MIPS: BMIPS: Add early CPU initialization code")
Signed-off-by: Jaedon Shin <jaedon.shin@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: Kevin Cernekee <cernekee@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 4.7+
Patchwork: https://patchwork.linux-mips.org/patch/16423/Signed-off-by: James Hogan <jhogan@kernel.org>