staging: most: core: show all linked channels

This patch is needed to have all linked channels being reported by the
show() function of the attribute file add_link. Currently user space can
only read back the latest link that has been established to a certain
channel.

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>