Fix detect logic, add default detect, make Dexcom use it

@nicolashery -- this makes it so that the detect call in dexcom works like other drivers, returning device ID and serial number, and makes it so that detect is called on all drivers.

I also gave detect a generic version (calling the first couple of driver functions). This has the side effect of spamming the console with the device scan -- but I didn't think that should be addressed in the PR. 
Should this still be called `detectFtdi`?