usb: nrf: Do not allow to clear STALL or dtoggle for ISO ep.

Isochronous transactions do not support handshake phase
so in particular the ISO endpoint cannot be STALLed.
Isochronous transactions do no support data toggle sequencing
and should only send DATA0 PID.

Taking into consideration those requirements do not try to
clear dtoggle or STALL an ISO Endpoint in nRF USBD driver.

Signed-off-by: Emil Obalski <emil.obalski@nordicsemi.no>