Trigger swupdate on usb media only

The swupdate usb udev rule was triggering on devices
which were not connected throug USB.

Signed-off-by: Leon Leijssen <leon.leijssen@topic.nl>
Reviewed-by: Stefano Babic <sbabic@denx.de>

recipes-support/swupdate/swupdate/swupdate-usb.rules

@@ -1,2 +1,2 @@
-ACTION=="add", KERNEL=="sd*", SUBSYSTEM=="block", ENV{ID_FS_USAGE}=="filesystem", TAG+="systemd", ENV{SYSTEMD_WANTS}+="swupdate-usb@%k.service"
+ACTION=="add", KERNEL=="sd*", SUBSYSTEM=="block", ENV{ID_BUS}=="usb", ENV{ID_FS_USAGE}=="filesystem", TAG+="systemd", ENV{SYSTEMD_WANTS}+="swupdate-usb@%k.service"