net: tftp: Fix TFTP Transfer Size data type

net: tftp: Fix TFTP Transfer Size data type

The TFTP transfer size is unsigned integer, update the data type
and print formating string accordingly to prevent an overflow in
case the file size is longer than 2 GiB.

TFTP transfer of a 3 GiB file, before (wrong) and after (right):
Loading: #################################################  16 EiB
Loading: ##################################################  3 GiB

Signed-off-by: Yuya Hamamachi <yuya.hamamachi.sx@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>