drivers: udc_dwc2: Optimize endpoint event clear

There is no point in calling k_event_test() to determine what events are
posted and then passing that value to k_event_clear(). Simply pass
UINT32_MAX to k_event_clear() and use the return value to slightly
reduce overhead.

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