iio: triggers: Make trigger ops structure explicitly non optional.

This structure has not been optional for a long time (if ever) but the
code implies that it is.  As we then use it later in a fashion that would
crash if it was in fact NULL, it's inconsistent so fix it up by removing
unnecessary checks.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>