device: s/dev_name/dev_id

The usage of dev_name is misleading, since it does not represent the
device name (struct device name field) but the name given to the defined
struct device. In practice, it is used as a kind of unique device
identtifier, so let's rename it to dev_id.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>