staging: most: core: replace struct most_inst_obj

This patch introduces struct interface_private as a replacement for
the struct most_inst_obj. This structure holds private data that is
only needed by the core module and will be accessed by a pointer from
within the most_interface structure. As a result of this replacement
the bus helper functions can be used to search for devices.

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