fw4: fix applying zone flags for source bound rules

fw4: fix applying zone flags for source bound rules

The rule parsing code failed to properly set the source zone flags for
rules requiring `${verdict}_from_${zone}` chains, causing those chains
to be missing, leading to errors when applying the ruleset.

Fix this issue by applying the flag to the correct property (source-
instead of destination flags).

Ref: https://github.com/openwrt/openwrt/issues/9686
Signed-off-by: Jo-Philipp Wich <jo@mein.io>