This is a doc-only patch which I hope will reduce the number of
spi_master controller driver patches starting out with a common
implementation bug.

(As in: almost every spi_master driver I see starts out with its
version of this bug.  Sigh.)

It just re-emphasizes that the setup() method may be called for one
device while a transfer is active on another ...  which means that most
driver implementations shouldn't touch any registers.
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>