iio:accel:kxsd9 fix missing mutex unlock

commit 0ee005c7dc2803125275e24598f0fb37775a6af3 upstream.

This will leave a lock held after reading from the device, preventing
any further reads.

Signed-off-by: Frank Zago <frank@zago.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>