security/openconnect-gui: fix fetch and bump PORTREVISION

I recalculated distinfo myself using make makesum, so it differs from
the distinfo given in the patch.

Note that version 1.5.3 is released for those who want to pick up this port.

PR: 245244
Submitted by: papowell@astart.com
Approved by: maintainer (pkbubaj)
MFH: 2020Q2