mtd: m25p80: make command buffer DMA-safe

spi_write() requires the buffer to be DMA-safe, kmalloc()
it seperately to ensure this.

Signed-off-by: Johannes Stezenbach <js@sig21.net>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>