microblaze: Fix put_user macro for 64bits arguments

For 64bits arguments gcc caused that put_user macro
works with wrong value because of optimalization.
Adding volatile caused that gcc not optimized it.

It is possible to use (as Blackfin do) two put_user
macros with 32bits arguments but there is one more
instruction which is due to duplication zero return
value which is called put_user_asm macro.
Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/include/asm/uaccess.h

@@ -189,7 +189,7 @@ extern long strnlen_user(const char *src, long count);
 
 #define __put_user(x, ptr)						\
 ({									\
-	__typeof__(*(ptr)) __gu_val = x;				\
+	__typeof__(*(ptr)) volatile __gu_val = (x);			\
 	long __gu_err = 0;						\
 	switch (sizeof(__gu_val)) {					\
 	case 1:								\