mfd: max8925-i2c: Fix variable length array Sparse warning

The largest byte size in use currently is 8.  Fix array size to 9.

drivers/mfd/max8925-i2c.c:40:33:
  warning: Variable length array is used
Signed-off-by: NLee Jones <lee.jones@linaro.org>

drivers/mfd/max8925-i2c.c

@@ -37,7 +37,7 @@ static inline int max8925_read_device(struct i2c_client *i2c,
 static inline int max8925_write_device(struct i2c_client *i2c,
 				       int reg, int bytes, void *src)
 {
-	unsigned char buf[bytes + 1];
+	unsigned char buf[9];
 	int ret;
 
 	buf[0] = (unsigned char)reg;