pci: Change error_report to assert(3)

Asserts are used for developer bugs. As registering a bar of the wrong
size is not something that should be possible for a user to achieve,
this is a developer bug.

While here, use the more obvious helper function.

Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
Message-Id: <20201015181411.89104-1-ben.widawsky@intel.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>