drivers: udc_dwc2: Avoid IN endpoint dequeue race

Flushing TxFIFO is racing with actual use of the TxFIFO. The software
controls only one side of the race (flush trigger) while the host
controls the other side. Therefore, locking interrupts before flushing
TxFIFO is not protecting against the race condition.

Disable the endpoint on dequeue to make sure that TxFIFO flushing won't
conflict with host actions (because the endpoint would be forced to NAK
the IN tokens before the TxFIFO is flushed).

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>