mtd cs553x_nand: use kzalloc() instead of memset

It's cleaner to use kzalloc() instead of zeroing out in a separate call
to memset().

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>