staging:iio: use spi->irq valid rather than querying available modes

Given these drivers only try to add the trigger if a valid
irq is present it is clearer to check the same condition when
deciding whether to remove it on a later trigger.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>