Fulfillment of the I2c trait contract

[ohunter]

This seeks to address the issue with the LinuxI2CDevice not correctly implementing the I2c trait as mentioned in rust-embedded/linux-embedded-hal#82. I am not sure whether the fix really belongs in this repo but since i2c_msg.flags isn't pub then it doesn't seem like there is another way. Setting the flag when the device doesn't implement the feature seems to have no effect which might lead to some confusion.

[nastevens]

Looks good now! Thanks for your efforts!

[nastevens]

Sorry, now rustfmt is unhappy :face_exhaling:

[nastevens]

Thanks again @ohunter, sorry it took a few tries to get things merged!

[ohunter]

No worries. I'm just happy to contribute 😀