tftp.c: fix CONFIG_TFTP_TSIZE for small files

CONFIG_TFTP_TSIZE should limit a tftp downloads progress to 50 '#'
chars. Make this work also for small files.

If the file size is small, i.e. smaller than 2 tftp block sizes the
number of '#' can get much larger. i.e. with a 1 byte file 65000
characters are printed, with a 512 byte file around 500.

When using CONFIG TFTP BLOCKSIZE together with CONFIG_IP_DEFRAG the
issue is more notable.

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>