cs: use device attributes instead of kobj attributes

In preparation for subsequent changes, switch to using device
attributes instead of kobj attributes. This fits well with the
implementation and makes it easy to support multiple instances
of the same device.

Change-Id: I3c3c58de9dba599bc44a57c2ddeb75100a4edf46
Signed-off-by: Pratik Patel <pratikp@codeaurora.org>