iio: inkern: fix a NULL dereference on error

In twl4030_bci_probe() there are some failure paths where we call
iio_channel_release() with a NULL pointer.  (Apparently, that driver can
opperate without a valid channel pointer).  Let's fix it by adding a
NULL check in iio_channel_release().

Fixes: 2202e1fc ('drivers: power: twl4030_charger: fix link problems when building as module')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>