scsi/gdth: fix crash in gdth_timeout if no gdth controllers found

If the gdth module is loaded (or compiled in), the gdth_timeout function
gets started even if no actual gdth controllers are found b the probing.

That ends up not only being unnecessary, but also causes a crash due to
the function blindly just trying to pick the first entry off the
"gdth_instances" list, and accessing it - which obviously doesn't work
if the list is empty!

Noticed by Ingo Molnar.

Tested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>