iio: tools: make generic_buffer look for "-trigger"

All the ST Sensors use the old "<foo>-trigger" rather than the
standard "<foo>-devN" new standard suffix for triggers. Now much
to do about it since it is ABI, but make the testing tools
recognize it too.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>