staging: at76_usb wireless driver

Add the at76_usb wireless driver to the staging tree while the
other kernel driver (out of tree) gets rewritten to use the internal
wireless stack.

This patch comes directly from the Fedora kernel tree, with only the
directory placement of the files changed.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>