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>

arch/mips/Kbuild

 # Fail on warnings - also for files referenced in subdirs
 # -Werror can be disabled for specific files using:
 # CFLAGS_<file.o> := -Wno-error
+ifeq ($(W),)
 subdir-ccflags-y := -Werror
+endif
 
 # platform specific definitions
 include arch/mips/Kbuild.platforms