staging: most: core: add comments to mutex and spinlock definitions

This patch adds a comment to the start_mutex and fifo_lock fields of
the most_channel structure definition.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Link: https://lore.kernel.org/r/1573566036-2279-1-git-send-email-christian.gromm@microchip.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>