Update two points in the SPI interface documentation:

- Update description of the "chip stays selected after message ends"
  mode.  In some cases it's required for correctness; it isn't just a
  performance tweak.  (Yes: to use this mode on mult-device busses, another
  programming interface will be needed.  One draft has been circulated
  already.)

- Clarify spi_setup(), highlighting that callers must ensure that no
  requests are queued (can't change configuration except between I/Os), and
  that the device must be deselected when this returns (which is a key part
  of why it's called during device init).
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>