ofono: Cancel pending SetProperty

ofono: Cancel pending SetProperty

If the cellular technolgy is disabled while ConnMan is connected
to a cellular network, the plugin gets first a network_disconnect()
call and shortly after that a modem_disable(). network_disconnect()
will set org.ofono.ConnectionContext.Active to false while this call
is pending modem_disable() tries to set org.ofono.Modem.Online to false.

There is no point in waiting for the first call to succeed or fail.
Just cancel it and set the modem offline.

Fixes BMC#24651.