mtd: nand: remove unused function input parameter

The nand_get_flash_type parameter "busw" input value is not used by any
branch, and it is updated before use it in the function, so remove it,
define the "busw" as an internal variable.

Signed-off-by: Cai Zhiyong <caizhiyong@huawei.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>