MIPS: Fix enabling of DEBUG_STACKOVERFLOW

Commit 334c86c4 ("MIPS: IRQ: Add stackoverflow detection") added
kernel stack overflow detection, however it only enabled it conditional
upon the preprocessor definition DEBUG_STACKOVERFLOW, which is never
actually defined. The Kconfig option is called DEBUG_STACKOVERFLOW,
which manifests to the preprocessor as CONFIG_DEBUG_STACKOVERFLOW, so
switch it to using that definition instead.

Fixes: 334c86c4 ("MIPS: IRQ: Add stackoverflow detection")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Adam Jiang <jiang.adam@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 2.6.37+
Patchwork: http://patchwork.linux-mips.org/patch/10531/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/kernel/irq.c

@@ -109,7 +109,7 @@ void __init init_IRQ(void)
 #endif
 }
 
-#ifdef DEBUG_STACKOVERFLOW
+#ifdef CONFIG_DEBUG_STACKOVERFLOW
 static inline void check_stack_overflow(void)
 {
 	unsigned long sp;