staging: most: fix tracking of MBO offset

This patch increments mbo_offs by the number of bytes that have
been copied and resets it in case a complete mbo has been transferred
to user buffer.

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