Linux: Fix unsubscribe from characteristic notifications not working

[aventari]

Fix for issue #366

Updated gattc_linux.go to properly unsubscribe from notifications using dbus call "org.bluez.GattCharacteristic1.StopNotify" which is the companion call to "org.bluez.GattCharacteristic1.StartNotify" that is being called when subscribing.

Tested working on Pi4

smoke-tests passed:

~/smoke/bluetooth $ make smoketest-linux
# Test on Linux.
GOOS=linux go build -o /tmp/go-build-discard ./examples/advertisement
GOOS=linux go build -o /tmp/go-build-discard ./examples/connparams
GOOS=linux go build -o /tmp/go-build-discard ./examples/heartrate
GOOS=linux go build -o /tmp/go-build-discard ./examples/heartrate-monitor
GOOS=linux go build -o /tmp/go-build-discard ./examples/nusserver
go: downloading golang.org/x/crypto v0.12.0
go: downloading golang.org/x/term v0.11.0
go: downloading golang.org/x/sys v0.11.0
GOOS=linux go build -o /tmp/go-build-discard ./examples/scanner
GOOS=linux go build -o /tmp/go-build-discard ./examples/discover
~/smoke/bluetooth $

[deadprogram]

Looks good thanks for working on this @aventari now merging!