tftp: Send an ack to the last data packet.

The way the tftp callback was written, it would claim success after receiving
the last data packet but wouldn't ack it back to the tftp server. This change
makes sure the last ack is actually sent, and makes sure the tftp client code
realizes a response was received so it doesn't resend the ack unnecessarily.

Change-Id: Ieb1cf4c245e5365921efb5191252e42919262d9d
1 file changed