cmd: nand: abstract global variable usage for dm conversion

nand_info is used all over the file so abstract it with
get_nand_dev_by_index() which will help for DM conversion.

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>