staging/most: fix return value for DIM_GetChannelState

Return NULL instead of 0 for invalid input.
Signed-off-by: NTomas Melin <tomas.melin@iki.fi>
Acked-by: NChristian Gromm <christian.gromm@microchip.com>
Signed-off-by: NAndrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/most/hdm-dim2/dim2_hal.c

@@ -889,7 +889,7 @@ struct dim_ch_state_t *DIM_GetChannelState(struct dim_channel *ch,
 		struct dim_ch_state_t *state_ptr)
 {
 	if (!ch || !state_ptr)
-		return 0;
+		return NULL;
 
 	state_ptr->ready = ch->state.level < 2;
 	state_ptr->done_buffers = ch->done_sw_buffers_number;