microblaze: Use unsigned type for limit comparison in cache.c

The patch removes warnings:
arch/microblaze/kernel/cpu/cache.c:146:14: warning: comparison of
unsigned expression < 0 is always false [-Wtype-limits]
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/microblaze/kernel/cpu/cache.c

@@ -140,10 +140,10 @@ do {									\
 /* It is used only first parameter for OP - for wic, wdc */
 #define CACHE_RANGE_LOOP_1(start, end, line_length, op)			\
 do {									\
-	int volatile temp = 0;						\
-	int align = ~(line_length - 1);					\
+	unsigned int volatile temp = 0;						\
+	unsigned int align = ~(line_length - 1);					\
 	end = ((end & align) == end) ? end - line_length : end & align;	\
-	WARN_ON(end - start < 0);					\
+	WARN_ON(end < start);					\
 									\
 	__asm__ __volatile__ (" 1:	" #op "	%1, r0;"		\
 					"cmpu	%0, %1, %2;"		\