iio: of_iio_channel_get_by_name() returns non-null pointers for error legs

Currently in the inkern.c code for IIO framework, the function
of_iio_channel_get_by_name() will return a non-NULL pointer when
it cannot find a channel using of_iio_channel_get() and when it
tries to search for 'io-channel-ranges' property and fails. This
is incorrect behaviour as the function which calls this expects
a NULL pointer for failure. This patch rectifies the issue.

Signed-off-by: Adam Thomson <Adam.Thomson.Opensource@diasemi.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Stable@vger.kernel.org