We need to be able to have a "SPI bus 0" matching chip numbering; but
that number was wrongly used to flag dynamic allocation of a bus number.
This patch resolves that issue; now negative numbers trigger dynamic alloc.
It also updates the how-to-write-a-controller-driver overview to mention
Signed-off-by: David Brownell <email@example.com>
Signed-off-by: Greg Kroah-Hartman <firstname.lastname@example.org>