pci: Correct cast for sandbox

This gives a warning with some native compilers:

cmd/pci.c:152:11: warning: format ‘%llx’ expects argument of type
   ‘long long unsigned int’, but argument 3 has type
   ‘u64 {aka long unsigned int}’ [-Wformat=]

Fix it with a cast.

Signed-off-by: Simon Glass <sjg@chromium.org>