sierra: fix APP1 PPP functionality

Unfortunately, Sierra secondary APP ports reply to +GCAP with
only "OK", and not their APP port number or model number.  So instead
of using +GCAP, we have to use ATI to get secondary port information.
This allows us to detect which port is the APP1 port that we can
potentially use for PPP, leaving the primary port available for
control and status.

Also, some modems have up to 3 or 4 APP secondary ports, which we
need to ensure aren't used as primary.  The previous check for +GCAP
handled that, but let's make it more explicit.

AT+GCAP reply:
OK

ATI reply:
Sierra Wireless, Inc.
C885
APP1
OK

See also: 3f3987e09ee762e48c1d53cb42a1288ce9f332cb (MM_06)

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