staging: most: dim2-hdm: fix race condition when closing a channel

This patch fixes race between the function poison_channel that clears
the state is_initialized and the tasklet function dim2_tasklet_fn that
checks the state is_initialized.

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>