staging: ccree: simplify access to struct device

Introduce a function to retrieve struct device from private
data structure in preparation to replacing custom logging
macros with proper dev_dbg and friends which require struct
device.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>