spi: limit reaches -1, tested 0

With a postfix decrement limit will reach -1 rather than 0, so the warning
will not be issued.

Also, add a cpu_relax() into the busy-wait loop.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Mariusz Ceier <mceier@gmail.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>