USB: gadget: udc: s3c2410_udc: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

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

drivers/usb/gadget/udc/s3c2410_udc.c:314:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/usb/gadget/udc/s3c2410_udc.c:418:7: 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>