staging: most: hdm-usb: synchronize release of struct buf_anchor

In case a channel that is going to be destroyed has been tagged as not
"healthy" by the function hdm_poison_channel() while the functions
hdm_write_completion() or hdm_read_completion() are being executed, they
race for destruction of buf_anchor.

This patch fixes the problem.

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>