staging: most: core: remove function get_channel_by_iface

This patch removes the function get_channel_by_iface that walks a list of
all registered interfaces and returns a pointer to a channel when matched.
Instead the private field of the interface structure is used to directly
access the channel via the id. The patch is needed to remove unnecessary
list traversing.

Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>