staging: most: core: rename device struct of core module

This patch gives the struct device variable of the core a more meaningful
name.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>