drivers: usb: udc: nrf: fix declaration after label

C language does not allow declarations after labels, only statements are
allowed. Add {} around the `NRFX_USBD_EVT_SETUP` case to fix build
issues (`error: a label can only be part of a statement and a
declaration is not a statement`)

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>