staging: most: i2c: remove redundant list_mutex

The elements of the dev->rx.list are consumed in the pending_rx_work and
populated in the function enqueue() that cancels the pending_rx_work.

The function enqueue() and poison_channel() do not race anyway.

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>