Older devices may crash if asked to connect right after sending the
PIN and unlocking the SIM; they simply stop responding to AT commands
around the first request for access technology and then reboot. A
delay seems prevents this behavior.
Since it's not uncommon to require a delay after SIM unlock, add one
for newer sierra_net devices as well, even though we're not quite
sure if they need one or not. It doesn't hurt, at least.
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