usb: dwc3-omap: Fix missing break in dwc3_omap_set_mailbox()

We need to break from all cases if we want to treat
each one of them separately.

Reported-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Fixes: d2728fb3 ("usb: dwc3: omap: Pass VBUS and ID events transparently")
Cc: <stable@vger.kernel.org> #v4.8+
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>