mtd: nand: pxa3xx: Use chip->cmdfunc instead of the internal

Whenever possible, it's always better to use the generic chip->cmdfunc
instead of the internal pxa3xx_nand_cmdfunc().
In this particular case, this will allow to have multiple cmdfunc()
implementations for different SoC variants.

Reviewed-by: Huang Shijie <shijie8@gmail.com>
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>