Make the spi_unregister_driver() code fit in with the rest of the header
file, and only do the action if the driver passed is non-NULL.

This also makes the code a line smaller.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
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>