usb: gadget: mv_udc: clarify a switch with an implicit fall-through

Rearrange statements in mv_ep_enable function so that it’s obvious
what the switch does and how zlt, ios and mult variables are
initialised.  Most notably, this gets rid of an implicit fall-through
so people don’t have to wonder whether it was intenional or not.

Addresses-Coverity-ID: 201385
Reported-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>