swupdate-usb: lessen hard requirement for swupdate-progress

If only swupdate-usb (and swupdate) is installed we are missing the
swupdate-progress package. This results in the following error:
Failed to start swupdate-usb@sda1.service: Unit
swupdate-progress.service not found.

Signed-off-by: Sean Nyekjaer <sean@geanix.com>

recipes-support/swupdate/swupdate/swupdate-usb@.service

@@ -1,7 +1,7 @@
 [Unit]
 Description=usb media swupdate service
 Requires=swupdate.service
-Requires=swupdate-progress.service
+Wants=swupdate-progress.service
 
 [Service]
 ExecStartPre=/bin/mkdir -p /tmp/%I