sierra: check for PS attach before dialing

Some devices (8775 specifically) get angry if you don't check for
a PS attach before dialing.  The next time they try to connect,
they'll continuously report "searching" as the registration status
and return CME ERROR 30 (No Network Service) for lots of requests.
So initiate a PS attach (which should return OK if the modem is
already attached) before dialing, and if that fails cancel the
dial attempt.

See http://mail.gnome.org/archives/networkmanager-list/2010-October/msg00072.html

"I talked about this problem with a contact at SierraWireless. He said that
a firmware upgrade will not help in this case. But what can help is checking
for PS attach and dial only if PS attach status is attached. Dialing without
PS attach can bring modem into unwanted condition that sometimes restart is
needed to recover."

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