staging/olpc_dcon: fix checkpatch warnings

The below checkpatch warnings was fixed,

-  WARNING: Prefer pr_debug(... to printk(KERN_DEBUG, ...
-  WARNING: Prefer pr_warn(... to printk(KERN_WARNING, ...
-  WARNING: Prefer pr_info(... to printk(KERN_INFO, ...
-  WARNING: Prefer pr_err(... to printk(KERN_ERR, ...

And added pr_fmt.

Signed-off-by: Toshiaki Yamane <yamanetoshi@gmail.com>
Acked-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>