disk: Fix detection of busnum

Unpatched version of libusb on linux platform does not set location, but
Debian's version of libusb has a patch which set it. For compatibility with
other systems without unpatched libusb version, read busnum from dirname
which seems to be always set.

Fixes: https://github.com/pali/0xFFFF/issues/5

src/disk.c

@@ -363,9 +363,17 @@ int disk_init(struct usb_device_info * dev) {
 
 		fclose(f);
 
-		if ( devnum != device->devnum || device->bus->location != busnum )
+		if ( device->devnum != devnum )
 			continue;
 
+		if ( device->bus->location ) {
+			if ( device->bus->location != busnum )
+				continue;
+		} else if ( device->bus->dirname[0] ) {
+			if ( atoi(device->bus->dirname) != (int)busnum )
+				continue;
+		}
+
 		if ( sscanf(dirent->d_name, "%d:%d", &maj2, &min2) != 2 ) {
 			maj2 = -1;
 			min2 = -1;