modemmanager: check during SIM unlocking whether initialization has been completed

modemmanager: check during SIM unlocking whether initialization has been completed

After the SIM has been successfully unlocked, it is initialized. This can
take longer on some modems, so we must wait until the modem is ready to
execute the next commands.

Otherwise the modem cannot be enabled and aborts with the following
error message:

error: couldn't enable the modem: 'GDBus.Error:org.freedesktop.ModemManager1.Error.Core.WrongState: modem in initializing state'

Signed-off-by: Florian Eckert <fe@dev.tdt.de>