tools:iio:iio_generic_buffer: fix trigger-less mode

Passing the trigger-less mode option on the command line causes
iio_generic_buffer to fail searching for an IIO trigger.
Fix this by skipping trigger initialization if trigger-less mode is
requested.

Technically it actually fixes:
7c7e9dad where the bug was introduced but as the window to the patch
below that changes the context was very small let's mark it with that.

Signed-off-by: Gregor Boirie <gregor.boirie@parrot.com>
Fixes: deb4d1fd ("iio: generic_buffer: Fix --trigger-num option")
Signed-off-by: Jonathan Cameron <jic23@kernel.org>