Minor updates to atmel_spi:

 - DMA:
    * Comments to explain the DMA policies
    * Report any mapping errors from spi_transfer()
    * Remove extra loop for DMA mapping

 - Diagnostics:  report minimum clock rate, if we need to reject a
   spi_setup() request because that rate is too low.
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Acked-by: NHaavard Skinnemoen <hskinnemoen@atmel.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>