cmd_net.c

The switch to strict_strtoul() was not done correctly - this fixes
the compile error when CONFIG_CMD_TFTPPUT is active.

There appears to very little use of strict_strtoul() in commands, so
it might be easier to switch to simple_strtoul(). If not, we should
decide the best way to use strict_strtoul(). For now I have added a
friendly message which will of course increase code size.

Signed-off-by: Simon Glass <sjg@chromium.org>