base-call: fix logic when accepting calls

Don't return FALSE when call is successfully accepted, otherwise the
caller will get very confused:

    ModemManager[19952]: <debug> [1528968478.344338] (ttyACM2): --> 'ATA<CR>'
    ModemManager[19952]: <debug> [1528968478.361986] (ttyACM2): <-- '<CR><LF>OK<CR><LF>'

    (ModemManager:19952): GLib-GIO-CRITICAL **: 11:27:58.387: g_dbus_method_invocation_take_error: assertion 'error != NULL' failed

And also, make sure the async task is always finished, even when
mm_base_modem_at_command_finish() returns an error which is not
MM_SERIAL_ERROR_RESPONSE_TIMEOUT.