imx: nandbcb: Fix potential overflow in nandbcb_set_boot_config

Fix Coverity Issue 9006656. In nandbcb_set_boot_config, an integer overflow
occurs, with the result converted to a wider integer type.

Signed-off-by: Ye Li <ye.li@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>