mfd: aat2870: Fix sparse error

Fixes the following error:
drivers/mfd/aat2870-core.c:296:20: error:
incompatible types in comparison expression (different type sizes)

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
diff --git a/drivers/mfd/aat2870-core.c b/drivers/mfd/aat2870-core.c
index 6f68472..14d9542 100644
--- a/drivers/mfd/aat2870-core.c
+++ b/drivers/mfd/aat2870-core.c
@@ -293,7 +293,7 @@
 	unsigned long addr, val;
 	int ret;
 
-	buf_size = min(count, (sizeof(buf)-1));
+	buf_size = min(count, (size_t)(sizeof(buf)-1));
 	if (copy_from_user(buf, user_buf, buf_size)) {
 		dev_err(aat2870->dev, "Failed to copy from user\n");
 		return -EFAULT;