proto: Clean up conversion warnings related to checksum macros

proto: Clean up conversion warnings related to checksum macros

These should not change any behavior, they mostly clarify
the used types and silence warnings, since these casts are
deliberate.

Change-Id: Ica721a51b00d5314125bcaf5a586e718c5982aef
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: MaxF <max@max-fillinger.net>
Gerrit URL: https://gerrit.openvpn.net/c/openvpn/+/1164
Message-Id: <20250926111726.153603-1-frank@lichtenheld.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg33218.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>