net: tftp: Fix incorrect tftp_next_ack on no OACK

When the tftp server did not send any OACK, the tftp_next_ack variable
was not set to the correct value . As the server was transmitting
blocks we generated a lot of 'Received unexpected block: $n, expected
$n+1' error messages. Depending on the timeout setting the transfer
could still complete though.

Signed-off-by: Harm Berntsen <harm.berntsen@nedap.com>
CC: Ramon Fried <rfried.dev@gmail.com>
Reviewed-By: Ramon Fried <rfried.dev@gmail.com>