[MIPS] ISA: Fix typo

Lost CONFIG_ prefix discovered by Robert P. J. Day <rpjday@mindspring.com>.
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

include/asm-mips/dma.h

@@ -74,7 +74,7 @@
  *
  */
 
-#ifndef GENERIC_ISA_DMA_SUPPORT_BROKEN
+#ifndef CONFIG_GENERIC_ISA_DMA_SUPPORT_BROKEN
 #define MAX_DMA_CHANNELS	8
 #endif