net: tftp: Avoid sending extra ack on completion

in tftpboot, if ack was already sent previously for this
packet, don't send again.

Fixes: cc6b87ec ("net: tftp: Add client support for RFC 7440")

Reported-by: Suneel Garapati <suneelglinux@gmail.com>
Signed-off-by: Ramon Fried <rfried.dev@gmail.com>
Tested-by: Suneel Garapati <suneelglinux@gmail.com>
Tested-by: Oliver Graute <oliver.graute@kococonnector.com>