net: Move the checksum functions to lib/

These functions are used by code outside the network support, so move them
to lib/ to be more accessible.

Without this, the functions are only accessible in SPL/TPL only if
CONFIG_SPL/TPL_NET are defined. Many boards do not enable those option but
still want to do checksums in this format.

Fix up a few code-style nits while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>