iio: percolate error if event fd fails

This makes the error from iio_event_getfd() percolate up
to userspace properly so we can know for sure there is no
events on this device (-ENODEV returned). Before this patch
we would bail out looking for the unsupported events on the
erroneous (negative) file descriptor.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>