staging: most: hdm-usb: add comment

This patch adds a comment to function hdm_configure_channel() to clarify
its execution paths.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>