staging: most: net: protect consistency of the state

This introduces the mutex that protects the consistency between the
tx.linked, rx.linked and the presence of the net divice.

Additionally, this patch optimizes the setup of the ch->linked in the
function aim_probe_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>