mfd: Fix aat2870 build failure for x86_64

Without this fix, we get:

error: call to ‘copy_from_user_overflow’ declared with attribute error:
copy_from_user() buffer size is not provably correct
make[3]: *** [drivers/mfd/aat2870-core.o] Error 1

And this was triggered by commit da417bacc9143b934f1a480a25d0fb2bb648a820

Signed-off-by: Jin Park <jinyoungp@nvidia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/aat2870-core.c

@@ -295,7 +295,7 @@ static ssize_t aat2870_reg_write_file(struct file *file,
 {
 	struct aat2870_data *aat2870 = file->private_data;
 	char buf[32];
-	int buf_size;
+	ssize_t buf_size;
 	char *start = buf;
 	unsigned long addr, val;
 	int ret;