ad9361-iiostream: Fallback to altvoltage for the channel name

Older versions of the kernel incorrectly used the 'altvoltage' channel type
instead of 'voltage'. Add support for this by falling back to 'altvoltage' if
the 'voltage' channel can't be found.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
iio_get_trigger(): Set trigger to NULL if no trigger is assigned

Instead of leaving the trigger uninitialized and returning an error set trigger
to NULL and return success if currently no trigger is assigned.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>