iio: inkern: fix a static checker error

Avoid this smatch error:
drivers/iio/inkern.c:751 iio_read_avail_channel_raw() error: double unlock 'mutex:&chan->indio_dev->info_exist_lock'

Fixes: 00c5f80c ("iio: inkern: add helpers to query available values from channels")
Signed-off-by: Peter Rosin <peda@axentia.se>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>