mtd: nand: pxa3xx: Move buffer release code to its own function

Create a function to release the buffer and the dma channel, thus undoing
what pxa3xx_nand_init_buff() did. This commit makes the code more readable
and will allow to handle non-DMA capable platforms easier.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>