staging: most: fix buffer size for DIM2

This patch reduces the DBR buffer size to prevent an overflow in the DIM2
module. It is needed, because the MediaLB hardware has problems with DBR
buffers that exceed the size of 255 messages.

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