net/bootp.c: fix tftp load if autoload environment var isn't set

Commit 09349866 (Put common autoload code into auto_load() function)
broke handling of autoload environment variable not being set.
The bootp/dhcp code will just keep on requesting IP address forever
and never start TFTP download.

Fix it by moving TftpStart() outside the conditional like it was before.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Tested-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Simon Glass <sjg@chromium.org>