mfd: omap-usb-host: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/mfd/omap-usb-host.c: In function 'usbhs_runtime_resume':
drivers/mfd/omap-usb-host.c:303:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (!IS_ERR(omap->hsic480m_clk[i])) {
       ^
drivers/mfd/omap-usb-host.c:313:3: note: here
   case OMAP_EHCI_PORT_MODE_TLL:
   ^~~~
drivers/mfd/omap-usb-host.c: In function 'usbhs_runtime_suspend':
drivers/mfd/omap-usb-host.c:345:7: warning: this statement may fall through [-Wimplicit-fallthrough=]
    if (!IS_ERR(omap->hsic480m_clk[i]))
       ^
drivers/mfd/omap-usb-host.c:349:3: note: here
   case OMAP_EHCI_PORT_MODE_TLL:
   ^~~~

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>