There are times when an assigned trigger to a device shouldn't ever
change after intialization.

Examples of this being used is when an provider device has a trigger
that is assigned to an ADC, which uses it populate data into a callback
buffer.
Signed-off-by: NMatt Ranostay <matt@ranostay.consulting>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>