drivers: udc_nrf: Do not submit buffer more than once

When the UDC buffer gets submitted it should no longer reside in the
endpoint queue. While this commit does not address the underlying issue
of not being able to start transfer for whatever reason, it prevents the
problem from cascading into buffer double completion (e.g. receive
buffer double free in UAC2).

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