Staging: rt3070: add support for Linksys WUSB54GC-EU v3

According to FCC (*1) and *2 there's really RT2070L inside, but according
to *2 RT3070 and RT2070 are compatible.

*1 https://fjallfoss.fcc.gov/oetcf/eas/reports/ViewExhibitReport.cfm?mode=Exhibits&RequestTimeout=500&calledFromFrame=N&application_id=327783&fcc_id=%27Q87-WUSB54GCV3%27
(look for "Internal Photos")
*2 http://rt2x00.serialmonkey.com/phpBB/viewtopic.php?f=5&t=5245



Signed-off-by: Marcin Slusarz <marcin.slusarz@gmail.com>
Acked-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>