cmd_nand: Update (nand_info_t*)nand after arg_off(_size) call

The arg_off() and arg_off_size() update the 'current NAND
device' variable (dev). This is then used when assigning the
(nand_info_t*)nand value. Place the assignment after the
arg_off(_size) calls to prevent using incorrect (nand_info_t*)
nand value.

Signed-off-by: Rostislav Lisovy <lisovy@merica.cz>