um: virt-pci: Fix 32-bit compile

There were a few 32-bit compile warnings that of course
turned into errors with -Werror, fix the 32-bit build.

Fixes: 68f5d3f3

 ("um: add PCI over virtio emulation driver")
Reported-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>