Fix conversion of AccessPoint flags to capabilities

Summary:
The value of Flags is a set of independant bits, so using a switch-case here
does not work.

For some reason NM 1.10.4 returns 3 as Flags, which makes this return None.

Test Plan: plasma-nm showed "unknown security type" before, now it's "WPA2-PSK" again.

Reviewers: Frameworks, jgrulich, kfunk

Reviewed By: jgrulich

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10997