Only defined signals can be written

This commits ensure that only known signals
could be written using the binding's api.
Before that you was able to wrote raw frames
on the CAN bus without any checks if this
was a known signals to the binding.

Bug-AGL : SPEC-2779
Bug-AGL: SPEC-2976

Change-Id: Ied6680e926f2a9c221fee31d8fb78d2d39c41132
Signed-off-by: Arthur Guyader <arthur.guyader@iot.bzh>
Signed-off-by: Romain Forlot <romain.forlot@iot.bzh>