staging: dwc2: set the driver name to "dwc2"

Previously, it was "dwc_otg", but this does not correspond to the
directory name and might cause confusion with the old out-of-tree
dwc_otg driver of which many versions circulate.

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>