staging:iio:trigger:bfintmr: Only enable timer when necessary

This patch hooks up the set_trigger_state callback for the blackfin timer
trigger driver and only enables the timer when a trigger consumer requests it to
be enabled. There really is no reason to keep the timer running and generate
interrupts if nobody is listening to them.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>