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 <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/spi/spi.c

···
  */
 int spi_register_master(struct spi_master *master)
 {
-	static atomic_t		dyn_bus_id = ATOMIC_INIT((1<<16) - 1);
+	static atomic_t		dyn_bus_id = ATOMIC_INIT((1<<15) - 1);
 	struct device		*dev = master->cdev.dev;
 	int			status = -ENODEV;
 	int			dynamic = 0;