Generic +CMER setting logic

[Aleksander Morgado]

Hey Dan, Carlo & everyone,

This patch series implements the checking of the AT+CMER command parameters so that we can enable or disable them without error. The parser only checks the 'mode' (1st field) and 'ind' (4th field) settings, as those are the ones we request to update.

Also modified the Cinterion and Telit plugins to avoid requiring custom AT+CMER settings (they had one because some devices didn't support ind=1, only ind=2). I've also seen the case where AT+CMER cannot even be disabled (mode=1 the only one allowed, as in the EM7345), and in that case we just skip the command.

Carlo, any chance you can make a quick test with these patches to see if they work as expected in the Telit modems? One of the changes I see is that the Telit plugin previously did the AT+CMER command only in the secondary port, while now we do it in both primary and secondary, not sure if that's an issue?

Cheers!

    [PATCH 1/4] helpers: new 'AT+CMER=?' parser
    [PATCH 2/4] broadband-modem: query +CMER format before enabling/disabling
    [PATCH 3/4] cinterion: remove custom +CMER enabling
    [PATCH 4/4] telit: remove custom +CMER enabling