nixos/connman: Fix assertion for networkmanager

Regression introduced by 5184aaa1

.

The fix was intended to remove the "x == true/false" assertions, but by
accident a "x == false" was made "x == true" instead of "(!x)".

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Reported-by: devhell <"^"@regexmail.net>