sr_usb_find(): Increase the 'bus' limit to 255.

On some systems it can happen that the USB 'bus' number is a lot larger
than 64, but sr_usb_find() currently errors out if it is > 64.

Example:
Bus 250 Device 006: ID 1ab1:04ce 1ab1 DS1000Z Series[...]

Increase that limit so that the code will work everywhere. This bus number
is queried via libusb_get_bus_number() which returns an uint8_t, so we're
limiting to 255 here.

Thanks to 'ssi' on IRC for reporting the issue.

src/usb.c

@@ -359,7 +359,7 @@ SR_PRIV GSList *sr_usb_find(libusb_context *usb_ctx, const char *conn)
 		return NULL;
 	}
 
-	if (bus > 64) {
+	if (bus > 255) {
 		sr_err("Invalid bus specified: %d.", bus);
 		return NULL;
 	}