tty: n_gsm: Fix DLCI handling for ADM mode if debug & 2 is not set

At least on droid 4 with control channel in ADM mode, there is no response
to Modem Status Command (MSC). Currently gsmtty_modem_update() expects to
have data in dlci->modem_rx unless debug & 2 is set. This means that on
droid 4, things only work if debug & 2 is set.

Let's fix the issue by ignoring empty dlci->modem_rx for ADM mode. In
the AMD mode, CMD_MSC will never respond and gsm_process_modem() won't
get called to set dlci->modem_rx.

And according to ts_127010v140000p.pdf, MSC is only relevant if basic
option is chosen, so let's test for that too.

Fixes: ea3d8465ab9b ("tty: n_gsm: Allow ADM response in addition to UA for control dlci")
Cc: [email protected]
Cc: Alan Cox <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Jiri Prchal <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: Marcel Partap <[email protected]>
Cc: Merlijn Wajer <[email protected]>
Cc: Michael Nazzareno Trimarchi <[email protected]>
Cc: Michael Scott <[email protected]>
Cc: Pavel Machek <[email protected]>
Cc: Peter Hurley <[email protected]>
Cc: Russ Gorby <[email protected]>
Cc: Sascha Hauer <[email protected]>
Cc: Sebastian Reichel <[email protected]>
Signed-off-by: Tony Lindgren <[email protected]>
Cc: stable <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

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