spi: fix double-free on spi_unregister_master

After 49dce689

, device_for_each_child
iteration hits the master device itself.  Do not call spi_unregister_device()
for the master device.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Acked-by: David Brownell <david-b@pacbell.net>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>