Modify spi_write_then_read() to use one transfer.  This speeds up all
callers, and is a minor code shrink.
Signed-off-by: NVernon Sauder <Vernon.Sauder@gmail.com>
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>