staging/most: fix return value for DIM_GetChannelState

Return NULL instead of 0 for invalid input.

Signed-off-by: Tomas Melin <tomas.melin@iki.fi>
Acked-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>