tools: kwboot: Fix sending retry of last header packet

After the trasfer of last header packet, it is possible that baudrate
change pattern is received, and also that NAK byte is received so that
the packet should be sent again.

Thus we should not clear the baudrate change state when sending retry
of that packet.

Move code for initializing state variables from kwboot_xm_recv_reply()
to kwboot_xm_sendblock().

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>