staging: most: remove unnecessary keep_mbo variable

The MBO pointer stacked_mbo and the boolean variable keep_mbo are
always changed together and therefore provide the same information.
This patch removes keep_mbo and uses stacked_mbo instead.

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