block: fix amiga and atari floppy driver compile warning

Geert, my crosstool don't produce warning below. I guess this has to do
something with compiler version.

- Geert noticed following warning during compilation.

  drivers/block/amiflop.c:1344: warning: ‘rq’ may be used uninitialized in
  this function
  drivers/block/ataflop.c:1402: warning: ‘rq’ may be used uninitialized in
  this function

- Initialize rq to NULL to fix the warning. If we can't find a suitable request
  to dispatch, this function should return NULL instead of a possibly garbage
  pointer.

- Cross compile tested only. Don't have hardware to test it.

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>