mtd: gpmi: allocate a proper buffer for non ECC read/write

The @data_buffer_dma buffer is used for non ECC read/write.

Currently, the length of the buffer is PAGE_SIZE, but the NAND chip may
has 8K page or 16K page. So we have to extend it for the large page NAND
chips.

The gpmi_alloc_dma_buffer will be called twice. The first time is to
allocate a temporary buffer for scanning the NAND chip; The second time
is to allocate a buffer to store the real page content.

This patch allocates a buffer of PAGE_SIZE size for scanning the NAND
chip when gpmi_alloc_dma_buffer is called the first time, and allocates a
buffer of the real NAND page size for the second time gpmi_alloc_dma_buffer
is called.

Signed-off-by: Huang Shijie <shijie8@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>