SPI ExclusiveDevice: access the bus object

[bugadani]

Sometimes it might be necessary/useful to access the SPI bus object that is held by an ExclusiveDevice. One such case might be adjusting SCLK speed based on some data read from the SPI device.

[Dirbaio]

This was discussed a bit in today's meeting. Sounds like a good feature to have. Could you send a PR?

[bugadani]

Definitely, if something like an inner_ref()/inner_mut() pair sound good. I'm on a vacation, though, so earliest is next week.

[Dirbaio]

I htink the convention is inner(), inner_mut()? We might want to do into_inner() too, to deconstruct it.