Normal TX handler for IN non-0 endpoints is called during
outgoing transfer or just after it was finished.
It may need to fill TX fifo with same data if TX_DONE is present
but ACK_STAT is not.
It may need to fill more data when called during transfer.
But it may also be called when STALL was sent.
In this case TX_DONE is set ACK_STAT is not, just like for packets
that were sent but no ACK was received.
Code was trying to send something again. There was nothing to send
so empty ZLP was scheduled for stalled endpoint.
This ZLP was later send to host where valid response was required.
This change checks if notification was for STALL endpoint and
does not try to fill TX FIFO in that case.
49aa0b72a8