SPI dynamic busid generation bugfix

Fix SPI dynamic bus ID assignment to start at 2^15-1 rather than a negative
number.  Valid bus ids are supposed to be positive, and are (now) stored in
an 's16' value.

Signed-off-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>