mtd: nand: pxa3xx: Remove unneeded internal cmdset

Use the defined macros for NAND command instead of using a constant
internal structure. This commit is only a cleanup, there's no
functionality modification.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>