staging: olpc_dcon: get rid of global i2c_client, create a dcon_priv struct

Rather than using the global i2c_client variable, create a dcon_priv
struct, store in the drvdata portion of the dev, and pass that around.

In order to access dcon struct from various callbacks, include
the reboot notifier and source switching work struct in the dcon struct.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>