um: virt-pci: Fix 32-bit compile

um: virt-pci: Fix 32-bit compile

BugLink: https://bugs.launchpad.net/bugs/1959376
[ Upstream commit d73820df6437b5d0a57be53faf39db46a0264b3a ]

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

Fixes: 68f5d3f3b654 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>