usb: gadget: at91_udc: change french comments to english

While being there, change C++ style comments to /* */.

Signed-off-by: Robert Schwebel <r.schwebel@pengutronix.de>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Cc: linux-usb@vger.kernel.org
Signed-off-by: Felipe Balbi <balbi@ti.com>