MIPS: Disable Werror when W= is set

Using any value for W= will lead to a ton of warnings which are turned
into fatal errors because MIPS adds -Werror to arch/mips/*.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: james.hogan@imgtec.com
Patchwork: https://patchwork.linux-mips.org/patch/15785/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>