mtd: nand: pxa3xx: Move DMA I/O enabling

Instead of setting info->dma each time a command is prepared,
we can move it after the DMA buffers are allocated.

This is more clear and it's the proper place to enable this, given
DMA cannot be turned on and off during runtime.

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>