Prevent get_pin_offset raising an exception

[jaydeetay]

... when executed on devices without the correct drivers installed. For example
when the documentation is generated by Sphinx (see: https://aiyprojects.readthedocs.io/en/latest/aiy.pins.html - it's
blank).

Not particularly happy with this solution - any better ideas?

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[jaydeetay]

@googlebot - I'm a Googler! Just registered my account today.