usb: gadget: atmel_usba_udc: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: at91_dt_defconfig arm):

drivers/usb/gadget/udc/atmel_usba_udc.c:329:13: warning: this statement may fall through [-Wimplicit-fallthrough=]

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>