wifi: Use g_signal_connect_object

The previous code used the g_signal_connect which could leave the signals
connected even after the NetDeviceWifi object was destroyed.

Automatic crash reports in Ubuntu indicate that this might be happening:
https://bugs.launchpad.net/ubuntu/+source/gnome-control-center/+bug/1718727
9 jobs for wifi-nm-signals in 18 minutes and 20 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Build
passed #83605
build

00:02:18

 
  Test
passed #83608
armv8

00:14:11

passed #83607
asan

00:02:02

failed #83609
allowed to fail
ppc64le

00:13:42

passed #83606
test

00:01:01

 
  Manual
manual #83612
allowed to fail manual
msan
manual #83610
allowed to fail manual
tsan
manual #83611
allowed to fail manual
ubsan
 
  Deploy
manual #83613
allowed to fail manual
flatpak
 
Name Stage Failure
failed
ppc64le Test
Full log written to /cwd/_build/meson-logs/testlog.txt
FAILED: meson-test
/usr/bin/python3 -u /usr/bin/meson test --no-rebuild --print-errorlogs
ninja: build stopped: subcommand failed.
Uploading artifacts...
./*.log: found 1 matching files
_build/meson-logs/: found 4 matching files
Uploading artifacts to coordinator... ok id=83609 responseStatus=201 Created token=C1ssk6KZ
ERROR: Job failed: exit code 1