net: Fix the displayed value of bytes transferred

In the case of NETLOOP_SUCCESS, the decimal value of the u32 variable
"net_boot_file_size" is printed using "%d", resulting in negative values
being reported for large file sizes. Fix this by using "%u" to print the
decimal value corresponding to the bytes transferred.

Fixes: 1411157d ("net: cosmetic: Fixup var names related to boot file")
Signed-off-by: Siddharth Vadapalli <s-vadapalli@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>