iio:common: added allocate and deallocate trigger functions when trigger is disabled.

This patch resolve a bugfix when driver is compiled without trigger.

Signed-off-by: Denis Ciocca <denis.ciocca@st.com>
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>