staging: most: core: fix sysfs attribute management

This patch creates a new attribute group to manage the attributes
of a registered aim module in sysfs and changes the show and store
functions accordingly.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>