econet: Add basic USB support with Mass Storage

econet: Add basic USB support with Mass Storage

The EN751221 has an XHCI that is compatible with MT7621.
While there is setup logic in the vendor code for both
EN751221 and MT7621, but MT7621 does not use it in mainline
or OpenWRT, and it appears to work correctly with EN751221.

Include SCSI / Mass Storage because many EcoNet devices
contain a builtin USB SD-Card reader.

Signed-off-by: Caleb James DeLisle <cjd@cjdns.fr>
Link: https://github.com/openwrt/openwrt/pull/20050
[Remove usb storage kmod from smartfiber_xp8421-b]
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>